zero carbon

Zero energy for all?

Zero energy for all?

From 2020 onwards all newly built or renovated houses in NI and ROI will have to comply with the Nearly Zero Energy Building (NZEB) standard. How that requirement will be put into practice remains unclear but there are some general rules we already know will have to be followed ..

Zero carbon targets and the construction industry

The new definiation of Zero Net carbon occurs after allowable solutions are introduced.

The new definiation of Zero Net carbon occurs after allowable solutions are introduced.

Zero carbon is great as a political aspiration but will it stack up effectively as a policy? Richard Hillyard examines Government aims to impose zero carbon targets on the construction industry. Back in July 2007 the Government published the Building a Greener Future statement. This policy document announced that all new build homes would be zero carbon from 2016.

The definition of zero carbon requires new dwellings to take into account:

  • emissions from space heating, ventilation, hot water and fixed lighting,
  • exports and imports from the development (and directly connected energy installations) to and from centralised energy networks.

Note:- Expected energy use from appliances is excluded from zero carbon definition.

By following this policy the Government expects new buildings to have net zero carbon emissions over the course of a year.

The definition of zero carbon consultation subsequently introduced by the government, sought views on the Government's proposals. This consultation ran from 17 December 2008 to 18 March 2009 and goes on to explain how to achieve net zero carbon emissions.

The Government also announced that from 2019 all non-domestic new builds will also be required to have zero net carbon emissions, with earlier dates for schools (2016) and public sector buildings (2018).

Wisely, the government set boundaries to what it meant by zero carbon. The embodied energy content of construction materials is not covered, and neither is the transportation of materials. Additionally, transport emissions associated with developments are not included as the government intends to deal with these through other policy instruments.

Given these omissions, it could be argued that Government's proposals do not equate to zero carbon. Even if it is not possible (nor cost-effective) to construct a building without generating any greenhouse gases, how far could we get by dramatically improving the efficiency and sustainability of construction methods?

In any case, does it really matter? Less than one per cent of the UK's existing building stock is replaced every year, and it's been estimated by the Department for Communities and Local Government (CLG) that 87 percent of the current housing stock will still be around in 2050. That means that the UK cannot meet its carbon reduction targets without a far-reaching retrofit programme for existing buildings.

The UK Green Building Council's proposal for a Code for Sustainable Buildings will play an important role in improving the focus on energy efficiency in existing buildings. But this is just one of a hierarchy of measures that the Government says will be needed.

The consultation document proposes a three-stage hierarchy for designers to achieve zero-carbon. The first step for energy efficiency requires compliance with Part L of the Building Regulations. This stage may also encompass other regulatory instruments, such as a mandatory requirement to design to Level 6 of the Code for Sustainable Homes.


The second stage proposed by Government is something called Carbon Compliance', which essentially is the use of on-site micro-energy generation. A report by the UK GBC Zero Carbon Definition Task Group believe over 80 per cent of homes in the UK to be unable to achieve zero carbon targets this way. The development of near-site and off-site low and zero-carbon energy generation is also being proposed.

Initially there were reservations over whether the use of biomass technologies could be included in the zero carbon strategy. However, the government appears to be in full support of using biomass systems both within new homes and as a source of direct heat from nearby off-site generation.

The third stage in the zero-carbon strategy is what is known as ‘allowable solutions', which is a buy-out fund or form of carbon offsetting through high quality international investment in low and zero carbon projects.

This third way will, it is believed, only be permitted where energy efficiency and carbon compliance are unable to be achieved totally through on-site and near-site measures achieve the goal of zero carbon - in other words the residual emissions.

The government is proposing a system of credits to permit off-setting to occur. Credits will be awarded to developments that have a range of energy-saving criteria. For example, energy-saving appliances and low and zero-carbon technology capable of exporting energy to the grid will earn credits to enable an offset of residual carbon emissions.

The government would prefer off-site low and zero carbon technologies to be included in this part of the hierarchy by feeding into the national grid.

So will the policy work? The first two parts of the hierarchy - energy efficiency and carbon compliance - are signs of forward thinking. With a few tweaks, off-site low and zero carbon energy generation could play an integral part of reaching the zero carbon target, but only if the contribution from the grid can be guaranteed to be clean.

Other questions remain to be answered. For example, with allowable solutions, will off-setting contribute to reducing carbon dioxide emissions enough for claims of zero carbon to stack up - not just initially but over a sustained period? Or is it, as some might argue, just a way of covering up holes in the system, and easing collective guilt?

Off-site low and zero carbon energy generation technologies sounds like reasonable measures, but if they are supplying to the grid as opposed to supplying directly to a development, what guarantees will there be that this clean energy will not be lost in the overall electricity generation? This is a key issue, especially when it's mixed with the output from the proposed eight new coal power stations (each potentially generating eight million tonnes of CO2 per year) that the Government is keen to build.

These questions highlight the credibility gaps that still exist between intention and delivery in Government's push for a low carbon and sustainable energy future. Whatever transpires following the zero-carbon consultation, tackling the issue effectively will not only significantly affect the environment, but also our pockets.

Zero carbon targets on the construction industry, [Online], Available: [18 March 2014].

CREST Centre - South West College - Passivhaus Certified, BREEAM excellent and Zero Carbon


Paul McAlister Architects were delighted to win an open tender for the provision of architect led design team services for the provision of the new CREST Centre for South West College Enniskillen. This project is one of the most sustainable projects in Ireland and will be the first commercial building in Northern Ireland that will achieve the Passive House Certification. The project is distinguished as it will achieve all of the following three sustainable credentials:

  1. Passivhaus Certified for Energy efficient envelope and ventilation system
  2. BREEAM excellent in terms of the BRE sustainable benchmark for UK commercials buildings
  3. The building will also be Zero Carbon, this means that the building can provide, by renewable energy, it own source of heat and lighting.

Whilst a combination of these sustainable criteria has been attempted in other parts of the UK, this will be the first example in Northern Ireland or Ireland and will become a benchmark building for sustainability.

South West College is one of six Further and Higher Education Colleges in Northern Ireland and was formed as a result of the merger of the former Colleges of Fermanagh, Omagh and East Tyrone on 1 August 2007.  South West College services the geographical area of Counties Tyrone and Fermanagh.  There are five campus locations at Cookstown, Dungannon, Omagh, two at Enniskillen (main campus and Technology and Skills Centre) and a number of out centres.  The College has approximately 500 full-time staff and a similar number of part-time staff and an annual budget of £39M. The college offers a wide range of vocational and non-vocational courses, training courses such as Training for Success, Steps to Work and provides a service to the community, local schools, business and industry.  It provides courses ranging from basic skills to Higher National Diploma and Degree Level programmes.

The CREST centre will provide industry Research & Development, demonstration and testing facilities for new renewable energy products and sustainable technologies. The facilities will be used by small companies within the region who have ideas for new products but who currently do not have the physical and/or technical capacity to develop, test and commercialise these. Within CREST facilities and staff will be accessible to develop, demonstrate and test new technologies and show how these can be integrated practically and sensibly to achieve energy savings. The CREST centre will form one component of a wider bid for European funding to support Research & Development in renewable energy and smart technologies within small businesses in the border regions of Northern Ireland, Ireland and western Scotland.

The CREST centre will comprise of three areas; the Pavilion, Research & Development Lab and Hub. The Pavilion will be newly developed and the Hub and Research & Development lab will be integrated into the existing Skills Centre building.

The Hub will form the central office area within the CREST centre and will comprise modern office and meeting space where the CREST team will meet with companies to discuss their requirements and outline the services available. Within the reception area of the Hub, real time visual data monitoring screens will be located. These will display easy-to-understand graphs and tables to analyse the energy performance of a variety of renewable energy installations (e.g. wind turbines, solar panels) located within the region and provide useful data such as information on CO2 savings and energy output. The CREST centre in Enniskillen will form the core of a larger network of satellite facilities in Sligo, Cavan and Dumfries in Scotland, and modern electronic communication facilities will be required within the Hub to link with these sites (e.g. Videoconferencing, web conferencing etc). The hub will be accommodated in the existing building which will require adaptation. Within the Research & Development lab a range of advanced prototyping, testing and development facilities will be available to enable staff at the centre to support companies with practical hands-on new product development. The Research & Development lab will comprise a large workshop facility which can be divided into smaller project workspaces. It is envisaged that some of the CREST projects will involve the development of highly innovative ideas for which IP protection will be required and thus some of the project workspaces will need to be screened from public access to prevent disclosure issues. Equipment within the Research & Development lab will include a laboratory scale wind turbine, biomass heating apparatus, heat exchangers, heat pumps, solar heating and photovoltaic and various monitoring, testing and ancillary equipment. This lab facility is to be accommodated within existing accommodation which will require adaptation. The Pavilion area of the CREST centre will provide demonstration and testing facilities to showcase innovative products and processes and the use of renewable technologies in construction. The Pavilion will be designed to engage industry through experiencing (seeing, touching) new sustainable technologies and materials allowing for a deeper engagement in and understanding of the techniques on display. Products and processes will include new construction technologies – building materials e.g. hempcrete, insulation etc. and renewable energy applications e.g. heat pumps, solar panels. It is planned that the Pavilion will be a dynamic facility which will allow for annual/biannual reconfiguration to include emerging technologies.


The 'zero carbon' definition

The definition of zero carbon has been under review and discussion since the initial proposal of the standard. The intention has always been that it would mean 100 percent reduction in net emissions relative to a dwelling compliant with the 2006 Building Regulations (England and Wales) - according to the initial definition, this was where any emissions created were offset by those 'saved' using on-site renewable capacity. Under what exact terms this will eventually be enforced and what the uplift in cost might be (and to whom) remains unresolved at this time. Carbon emissions have been separated into what are now termed ‘regulated’ and ‘unregulated’ carbon emissions. Regulated emissions are those from fixed building services, i.e. heating, ventilation and lighting; unregulated emissions are those relating to energy used by the building occupants, e.g. from cooking or electrical appliances. The initial plan was that 100 percent reduction in both would be the target - the L6 standard. Concerns about whether this was a realistic target in practice led the government in 2011 to alter the definition of zero carbon to include only the ‘regulated’ carbon emissions. This means that zero carbon can now refer to either Level 5 or Level 6 of the Code for Sustainable Homes.

Building regulations compared to CSH
Building regulations compared to CSH

The Zero Carbon Hub, a public private partnership, has been working since 2008 to support the delivery of zero-carbon homes, and this includes the development of a final definition for zero carbon. The Hub's extensive work has included publishing various advisory papers and carrying out useful consultations.

Allowable solutions

Achieving 100-percent reduction in carbon emissions, even if from regulated emissions only, involves the significant use of on-site renewable energy sources. The practicalities of having enough roof space, not to mention the cost burden, has led to a further strategy being introduced - allowing a certain proportion of the emissions reductions to derive from off-site sources or what are termed 'allowable solutions’. What exactly these will be or how they will be delivered is again unknown, but will most likely involve a payment towards the cost of introducing carbon-saving projects. The proportion in carbon reductions that will have to be achieved by the house itself is termed 'carbon compliance'. The split between these two is a matter of further debate. Initially it was thought that 70 per cent of the emissions reduction would be met by carbon compliance and 30 per cent by allowable solutions. An excellent paper by the Zero Carbon Hub highlights that this still remains very demanding and unrealistic for some house types. The recommendations in that report suggest the following carbon compliance levels:

  • 60 per cent for detached houses
  • 56 per cent for attached houses
  • 44 per cent for low-rise apartment blocks.

Describing Allowable Solutions

‘Allowable Solutions’ has been a useful catch-all term for any approved carbon-saving measures that would be available to developers from 2016 to allow for the carbon that they would not normally be required to mitigate on site through Carbon Compliance (see illustration).

The expectations that have become associated with Allowable Solutions are:

  1. That the developer would make a payment to secure emissions reductions through (largely) near-site or off-site, carbon-saving (Allowable Solutions) projects;
  2. That, independent of the developer, there would be an opportunity to aggregate a number of Allowable Solutions payments to deliver larger scale carbon-emission reduction projects;
  3. That Allowable Solutions would be affordable and (per unit of carbon) would cost, at least initially, less than Carbon Compliance;
  4. That wherever possible, Allowable Solutions would be linked with local projects that would bring local benefits.
Zero carbon comliance
Zero carbon comliance

The carbon compliance percentage achieved on any project will be determined by the efficiencies achieved by the building fabric and building services plus any on-site low or zero-carbon energy source.

In the original proposal for the zero carbon standard there were no specific building fabric efficiency targets. This was addressed in the (2010) Code for Sustainable - Homes: Technical Guide, which set a new criterion, the Fabric Energy Efficiency Standard (FEES), for the dwelling's space heating. Since the zero carbon standard for new homes is not to be enforced until 2016, FEES is likely to be developed further before then. At the time of writing, it helpfully introduces a Passivhaus-style space heating energy target - for the first time moving away from a carbon emissions measurement.

We feel this is a positive development. The lower the space heating target you can achieve, the less you need to make up to meet the overall carbon compliance percentage. Reducing the on-site carbon emissions through excellent fabric performance is also key to reducing the need for additional and expensive on-site energy production. Energy generating systems will also have a shorter expected life (a solar hot water system, for example, will last 10 to 25 years) compared with the general building fabric (average 60 years), so prioritising investment in fabric makes good sense.

For the zero carbon level, the FEES target is (currently) 39-46kWh/m2.a. The range reflects different house types - for example, it is easier to improve performance on a mid-terrace than on a detached house. The UK's Standard Assessment Procedure (SAP) method for measuring floor area is more generous than that of Passivhaus which means that the zero carbon target is actually equivalent to over 50kWh/m2.a in Passivhaus terms. The consultation for the next updated Building Regulations, Part L (2013) is also proposing an interim Target Fabric Energy Efficiency (TFEE) requirement for new dwellings, of 43-52kWh/m2.a, adopting the same measuring standard as the zero carbon FEES levels. As yet there is no final decision on the standard they propose to adopt - the TFEE or full zero carbon proposed FEES levels. Whichever is adopted will then also link into Level 4 of the CSH. The interim TFEE would apply from 2013 until 2016.

Fabric performance and ventilation strategies

The zero carbon space-heating target range is clearly less stringent than that of Passivhaus (at l5kWh/m2.a), and this reflects a reluctance to limit ventilation strategies to MVHR. There is a body of opinion that advocates natural ventilation solutions such as passive stack ventilation(a whole-house ventilation system that uses naturally occurring pressure differences to draw air in through trickle vents in windows and then up and out through ducts in the kitchen and bathrooms). This necessitates higher air-leakage rates, which come with a significant energy penalty. The air-leakage benchmark for zero carbon is being currently mooted at an air permeability of 3m3/hr/m2, This is approximately equivalent to 3ach (air changes per hour) for an average-sized house. There has also been discussion regarding the capability of the general construction industry to achieve very low air change rates, i.e. below 3ach. This compares with the Passivhaus standard of 0.6ach - again, a far more stringent standard.

Without mandatory fabric energy targets, there was an early tendency for those looking to meet the zero carbon standard to rely on relatively complex and hard-to-maintain low-carbon heating solutions to achieve the carbon emission reductions. This concentration on renewable technologies can easily lead to a less informed focus on building fabric, which will increase other risks. By beginning to increase levels of insulation in our homes and making them much less 'leaky', we are changing the way they physically behave, and an understanding of this is key. Making radical changes without understanding carries four major risks:

  • reduced indoor air quality (IAQ)
  • moisture within the fabric causing mould and deterioration of materials (and exacer­bating the risk of reduced IAQ)
  • unacceptable overheating in summer
  • underperforming buildings (in energy terms).

Introducing FEES will - rightly - refocus on the building fabric, but this will need to be coupled with appropriate modelling methods and training so that these changes in building construction do not lead to the risks just noted. Linking the required measurements to as-built performance (measuring real performance after occupation), rather than design performance (using energy modelling software during design), as now recommended in the zero carbon consultations, will assist with this.

A zero-carbon Passivhaus

There is a misconception that Passivhaus "will only get you to L4 (Level 4) of the Code for Sustainable Homes". If you built a Passivhaus and chose not to address any of the other sustainability criteria assessed in the CSH, then this might be true. In particular, Passivhaus does not include the water usage criteria demanded by Level 5 and Level 6. However, if water usage is addressed in a Passivhaus design, it should comfortably meet Level 5.

In fact it should be easier and cheaper to meet a zero carbon standard in a Passivhaus than in a structure that is not designed as a Passivhaus. This is simply because the building fabric of a Passivhaus leads to the exceptionally low energy requirement for space heating, even relative to a zero-carbon house

The Passivhaus standard also includes a total primary energy demand of 120kWh/m2.a, which will also help to achieve zero carbon because it encourages efficient energy use across all electrical appliances and uses within a building.

Together, this means that if regulated and unregulated carbon emissions are taken into account, a Passivhaus can reach a zero carbon level with minimal additional renewable devices. If regulated emissions only are considered (the current zero carbon definition), a Passivhaus has been shown to achieve a 65- to 70-per-cent reduction in regulated carbon emissions compared with a compliant Part L (2006) dwelling, when calculated using the PHPP, without the use of any on-site low- or zero-carbon energy provision. This meets all the current zero carbon 'carbon compliance' recommended emissions reductions (44, 56 or 60 per cent). In making a comparison between Passivhaus and zero carbon, it should not be forgotten that, by using the PHPP and applying Passivhaus methodology, a much more reliable prediction of real-life performance is achieved. And the Passivhaus also addresses summer overheating risks and IAQ (Internal Air Quality) much more reliably than a non-Passivhaus low-energy building.

Even where a project aim is to achieve zero carbon, it is worth giving serious consideration to using the PHPP and applying Passivhaus methodology and principles to the building fabric design, i.e. to meet or, even better, to exceed, the FEES target. There are many efficiency gains and no conflicts.

Broader sustainability criteria

As we have seen from the issues relating to ventilation strategies discussed above, the Passivhaus focus on building fabric performance, and scientific research into and testing of this, is critically important. The fact that other sustainability criteria are not included as part of the Passivhaus standard has ensured that this focus has been maintained. This is not to say, however, that other sustainability criteria apart from carbon emissions (which zero carbon focuses on) and energy in use  (which Passivhaus focuses on) are not important: the fact is that Passivhaus buildings can be made from many different materials and construction methods (both lightweight and heavyweight), and the Passivhaus standard is perfectly suited to combining with more diverse sustainable assessment systems. Unfortunately, official certification using two different assessment methods on one building will have cost implications, but in technical terms there is no inherent incompatibility between Passivhaus and other systems, such as the CSH or BREEAM, or the US systems LEED and HERS. Other standards can usefully widen the Passivhaus approach to consider some broader sustainability issues, including recycling, water management and 'Lifetime Homes' recommendations.

On-site low- or zero-carbon energy

While a zero-carbon Passivhaus may not require any or only minimal on-site or zero-carbon energy for carbon compliance, you may still want to consider such options. If the aim is only to meet the zero carbon FEES targets (not the full Passivhaus target), then some on-site or zero-carbon energy will be essential. (‘Zero-carbon energy’ normally refers to biomass fuel, while 'on-site' refers to energy generation.)

Carbon compliance
Carbon compliance

What exactly is a Zero Carbon Home?

The intention to offer a stamp duty tax break to zero carbon houses in Britain has been announced, but what exactly is the definition of as a zero carbon?

The aim of the relief is to ensure that on average over the course of a year the homes are zero carbon. In other words, they will not be required to be zero carbon the whole time, but the import of grid power and export of renewable power should at least balance over the course of a year.

Seems fair enough. It continues…

It will also be necessary to ensure that the fabric of the building significantly exceeds the standards currently required by Part L of the Building Regulations 2000 (as amended). The requirement will be that the “Heat Loss Parameter” (covering the walls, windows, air tightness and other elements of the building design) is no more than 0.8W/m2K. This standard will mean that space heating requirements are no more than 15kWh/m2 per annum.

Heat Loss Parameter

“Heat Loss Parameter” is a phrase I wasn’t familiar with but it’s embedded in the SAP calculations. It’s defined as the specific heat loss for the whole house divided by the internal floor area. The figure 0.8 is very exacting, equivalent to constructing to PassivHaus standards, which is where the 15kwh/m2 figure comes from. In fact, I have learned from the Treasury that the figure was defined by taking the PassivHaus standard and playing it through the SAP calculations on a notional house. To get down to 0.8, you need to have super insulated walls, floors and roofs, triple glazing, virtually no unplanned air loss and mechanical ventilation with heat recovery.

Heat and power for space heating, hot water and lighting must be generated either in the home or on the development or through other local community arrangements (including district heat and power) and must be renewable (i.e. non-fossil fuel) energy.

Here it departs from PassivHaus standards and starts to require renewable power input. But what exactly does it mean by other local community arrangements? It is a frustratingly vague phrase.

What about power use over and above space heating, hot water and lights? It goes on:

A zero carbon home is also required to have zero carbon emissions from use of appliances in the homes (on average over a year).

There follows a table which specifies how much additional power you need to generate depending on the overall floor area of the house. Bigger homes, over 150m2 floor area, need to generate 16.54kWh/m2 to cover cooking and appliances. So a 200m2 home, the typical size built by a UK selfbuilder, would need to generate 3308kWh/annum to cover this load.

This additional power must be renewable power produced either within the area of the building and its grounds, elsewhere in the development, or elsewhere as long as the developer has entered into arrangements to ensure that the renewable generation is additional to existing plans.

Now there appears to be an important distinction here. Appliance power, it would seem, can be offset by the developer, whereas power for space heating, hot water and lighting must be site generated, or at least community generated. It’s not exactly clear what the distinction is, or what is meant by entering into an arrangement but the fact that it exists is confirmed by Cl 18 which states - The amount of such additional power can be reduced by any surplus from the arrangements to meet zero carbon on heating, hot water and lighting.

So how does one square this? In what sense can one type of electricity be subject to less stringent requirements than another? It’s something that appears nonsensical. Perhaps I shouldn’t be surprised, because its followed by the silliest clause of all of them.

Qualifying homes will not be permitted to be connected to the gas main. Mains gas is by far and away the most efficient way of burning fossil fuel. So where on earth is the logic in banning it? Especially as there is nothing to stop you using bottled gas for the same purpose? Or of course cooking with grid-delivered electricity, which releases around two and a half times more carbon dioxide than mains gas. In fact, I am pretty certain you could install an electric Aga, just about the most energy hungry household appliance on the planet, and it wouldn’t effect your zero carbon status one jot. I am afraid the Treasury is shooting itself in the foot here.

So how would you go about building a zero carbon house?

First you would need to construct something close to a PassivHaus, a super insulated shell with triple glazing and mechanical ventilation with heat recovery. The PassivHaus rules are quite prescriptive and it’s not clear just how closely you would have to adhere to them.

Then you would have to develop a strategy to provide renewable power to cover your anticipated demand for space heating, hot water, lighting, cooking and electrical appliances. You would need to carry out a heat loss calculation, using the government SAP rating scheme, to work out what this figure would be but you can assume that overall it is likely to be around 50kWh/m2/annum, or 10,000kWh/annum for a 200m2 house, of which perhaps as much as 30% can be offset — although what counts as an acceptable offset isn’t detailed. You could achieve this with:

  • 6kW wind turbine, mounted in an appropriately windy spot, cost around £15,000 after grant deductions or
  • around 80m2 of photovoltaic solar panels, cost around £30,000 after grants.

Hot water and space heating demand could be met by a biomass or pellet boiler, cost around £10,000 and/or solar hot water panels, (cost around £4,000 but will only deliver part of the load.) But neither of these options produce electricity so presumably there would have to be additional installations to cover electricity demand as well. It’s not exactly clear. Alternatively, heat pumps could be used to reduce the heating demand, but they don’t generate any power either, so it’s hard to see quite how they fit into the picture. One form of micro generation, the CHP plant, can’t be used at all, despite the fact that they do generate electricity, because the current models use mains gas, which is of course banned! Oh the joys of joined-up thinking!

Presumably you would be able to mix and match these technologies so that you could for instance use a heat pump combined with a solar hot water panel to reduce your demand and then fit a smaller PV array to make up the difference and then offset the appliance load. But what isn’t explained is how each individual scheme will be judged: there is a lot of fairly complex carbon accounting going on here and you only have to vary the assumptions slightly to get very different results. Who is to say House A qualifies as zero carbon whilst house B doesn’t? I have no idea.

Finally, what’s the tax break worth? As I understand it, the zero carbon house will be exempt from stamp duty only on its first sale and only up to a maximum of £15,000. If you sell the house for over £500,000, which is where the top rate of 4% stamp duty starts, then you would subtract the £15,000 from the duty normally payable. The beneficiary of this largesse is therefore not the developer but the first purchaser, who pays the stamp duty in the normal course of events, although the developer may gain by being able to sell the zero carbon house for a little more than the market would normally bear, because of the stamp duty holiday. Tax breaks should never be sniffed at, but the reality is that the cost of uprating a normal house to a zero carbon house will invariably be considerably more than the value of the tax break.

One supplementary point worth noting: Tony Verran of HM Treasury, in commenting on first draft of this piece, has indicated that selfbuilders will not be eligible for the tax break because it will only cover the first use of the home for residential purposes. It doesn’t cover bare land and it won’t cover a sale made to a second user, even if it’s technically a first sale.