A customer asked me if they should get the SAP calculation completed before they started building. The answer is yes. You should have a SAP calculation while you are still at the planning stage before you start building-I would advise even before you buy any materials. It is very likely that your building, as you’ve designed it, isn’t compliant and that the SAP Assessor will need to give you a great deal of advice about how to put it right.
Many customers underestimate the difficulty involved in getting the house to pass. Surely the building will pass if it is well heated and well insulated? Modern Part L1A is an extremely high standard. 2005 Building Regulations required a common sense approach to produce a well heated, well insulated building. Part L1A 2013 has CO2 targets 28% below SAP 2005. At this sort of standard it is extremely difficult to predict the performance of a building simply by looking at the quality of its heating systems and insulation. The only way you can be absolutely sure it will pass is to do the SAP calculation.
There are government U value tables. Wouldn’t you be all right if you just used those? The government U value tables contain maximum values that the thermal element could be, for example, the house would definitely fail if the wall had a U value greater (worse) than the one in the table. The SAP calculation looks at the whole house as it works together. It is unlikely the house would pass with the wall at the U value it is in the table unless everything else was exactly right. If you built a whole house using the U values in the government table, as many of my customers have done, it would definitely fail.
Making changes to a building when it is completed can be expensive. It involves increased labour costs and may include an extra layer of insulation on the inside walls or additional solar PV panels. These may have been avoided with careful design at an early stage. Get the SAP calculation done first to save yourself the nightmare of having a building which needs thousands of pounds worth of work done on it when it has actually been completed.