A good air tightness result is a figure that enables your SAP calculation to pass. The better the final air tightness test result, the better the energy calculation, which in turn means the final EPC (Energy Performance Certificate) will have a higher rating. The better the result and tighter the building, the more efficient your property will be. This will cost you a lot less to run in heating and lighting bills, and energy efficiency will be better. 

The EPC will provide you with an estimate of what those costs will be. The building should be tested as soon as it is complete, as all the final finish has been completed and the best chance of a successful result is possible. 

The more advanced the build and final finish, the better the result will be. Most leakage points will be sealed, the better the finish, the better the result. 

The air tightness test of a building is guided by a set target within the energy calculation (SAP or SBEM). The overall calculation determines the target figure. This target figure must be met or bettered to pass the air tightness test. 

Currently, the maximum leakage rate allowed within the energy calculation is 10m3/(hr*m2) @50 Pa. Although the maximum leakage rate allowed within the energy calculation is 10m3/(hr*m2) @50 Pa, this number is usually lower, as it is common for the energy assessor to lower the target score to enable the calculation to pass. 

Air tightness testing is a mandatory requirement of Part L of the building regulations for a new build property, whether residential or commercial. All new build buildings must be tested.