The 'zero carbon' concept introduced by the UK government is mainly of a fiscal nature. Although it has technical ramifications it is not technically prescriptive.
All details about the fiscal measures and the requirements a house must fulfil in order to be considered a 'zero carbon home' have not yet been published.
The bulk of the information available can be found in Budget Notice 26 to the 2007 budget, Stamp Duty Land Tax: Relief for New Zero Carbon Homes published on March 21st 2007.
According to this document SDLT relief will be available on the first sale of zero carbon homes up to a purchase price of £500.000 whereas the tax will be reduced by £15.000 for more expensive homes. The measure came into force on October 1st 2007 and is scheduled to expire on September 30th 2012.
In order to qualify a home must be 'zero carbon' on average over the course of a year. Whether or not this is the case will be determined by a SAP calculation, SAP being the Government's Standard Assessment Procedure for Energy Rating of Dwellings. The current version of SAP is probably not very suitable for this purpose, and it will require considerable improvement to make it so. A better choice would be the Passivehouse Project Package, PHPP, the calculation principle used to calculate the energy consumption of passivehouses. The 2007 edition of this software contains full UK climate data, and as opposed to SAP, is specifically designed to be able to calculate ultra-low energy buildings. In fact 'zero carbon' homes will need to have a Heat Loss Parameter not exceeding 0.8W/m2K and, like a passivehouse, a space heating requirement not exceeding 15kWh/m2a.
However, the government's 'zero carbon' concept does not only depend on the fabric of the building but also on how it is energy-supplied. Not only will a building in order to qualify need to reach 'zero carbon' for energy consumed by heating, lighting and hot water provision (according to the SAP computation) but heat and power must either be generated 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.
On top of this, and as something new, a 'zero carbon' home must also have zero carbon emissions from the use of appliances in the home, again on average over a year. SAP is supposed to be upgraded to be able to take this into account in the future. Until this happens a home must supply an annual amount of renewable electricity depending on the floor area, ranging from 24.97kWh/m2a for a floor area of up to 89m2, to 16.54kWh/m2a for homes with a floor area above 150m2. The 'zero carbon'ness pertains to the overall sum of these factors and a key clause is that this renewable energy can be produced elsewhere insofar as the generation is additional to existing plans. For the ordinary home owner this is the get out of jail free card.
It is also planned to incorporate wind power and micro generation equipment into SAP. Unfortunately neither are fantastic solutions. Wind power, at least on a small scale, is not viable as a reliable main power supply solution, and for reasons of ordinary physics you cannot avoid generating too much heat for a passivehouse in the otherwise attractive micro combined heat and power plants (CHPs).
Finally, a 'zero carbon' home cannot be connected to the gas mains. This is an understandable decision although it might be wise to bear in mind that the gas distribution network is admirably suitable for the distribution of hydrogen, something which may become relevant in the not too distant future.