Approved Document L1A demands testing of selected samples of dwellings by "type". This Appendix to ATTMA TS1 defines these "Dwelling Types" Various generic forms of dwelling are considered as separate discreet types. Examples include:

• Detached
• Semi detached
• End-terrace
• Mid-terrace
• Ground-floor flat
• Mid-floor flat
• Top-floor flat

The number of storeys will also define different dwelling types, i.e. 1, 2, 3, etc. For dwellings to be considered to be of the same type:

They must contain the same construction details for each of the main elements, i.e. walls, floors and roofs. They must have similar floor areas. Small variations in gross floor area do not constitute a different dwelling type. For the purposes of this Technical Standard the difference in gross floor area between the largest and smallest within a dwelling type should be no greater than 15%.

They must have a similar number of significant penetrations (SP) defined as the sum of the total number of window frames and entrance door frames (including patio door frames) in the external facade. Flues are also counted as significant penetrations. A dwelling can't be considered as the same dwelling type if the total number of significant penetrations varies by more than +/-1. (For example, if a dwelling type contains 6 SP's then dwellings with 8 or more SP's can not be considered as the same dwelling type neither can 4 or less SP's be considered as the same dwelling type).

For the purposes of this Technical Standard a cold roof construction will be considered as a different dwelling type from a warm roof construction, since in the latter case the loft space will be included in the airtightness tests. Similarly a cold floor, flat above an access road will be considered as a different dwelling type.

Where there are a number of dwellings within a dwelling type, there may be variations in their design air permeability due to SAP calculation differences. In order for all those dwellings within that type to conform to Part L1A then the lowest dwelling design air permeability must be used as the acceptance criteria, regardless of which plot is tested. Alternatively, the dwelling type should be subdivided according to their design air permeability resulting in more dwellings of that type being tested

Reference ATTMA Technical Standard 1 (TS1) 2007 Edition