• Yes for commercial buildings
• No for dwellings

For commercial developments, where testing is required, each individual building unit will need to be tested ie if the ‘unit’ is not connected to adjoining units and intended to be occupied/operated separately.

However, in practice we have seen similar units being covered by a sample.

On residential developments a sampling rate applies depending on:

1. Dwelling Types
2. Adoption of ‘Accredited Construction Details

1. Dwelling Type

For housing developments, a sample of units from different ‘dwelling types’ are tested to prove compliance. Separate blocks of flats are considered as different developments.

Dwelling types are defined based on:

1. Position – this is likely to be:

• Detached house (no party walls)
• End terrace/sem-detached house (1 party wall)
• Mid terrace house (2 party walls)
• Ground/bottom floor flat (party floor below - may be over car parking, drive through, etc.)
• Intermediate floor flat (2 party floors)
• Top floor flat (party floor below)

    2.   Construction type – whether they are traditional/masonry, timber frame, steel frame, etc. ie 2 identical detached houses, 1 of timber frame and 1 of traditional construction would be considered different dwelling types.

Other factors may also effect dwelling types based on ATTMA TS1:

• Size: Part L makes reference to size but fairly vaguely: ‘small changes in ground floor area do not constitute a change in dwelling type’. ‘Small’ is not defined so is open to interpretation. Ultimately, Building Control will define what constitutes a significant change in size but as a basis for estimating it is safe to assume this will not effect the dwelling types.  ATTMA suggest 15% variation.
  
• Storeys: There is argument to say that storey height does not effect dwelling types if there is no potential air leakage paths where partition floors meet external walls. However, in practice bungalows are often seen as different. 2.5 storey units are likely to require different detailing to 2 or 3 storey so are more likely to be treated as a separate type.
• Significant penetrations: ie number of windows, doors and flues.
• Variations in wall, roofs and floor constructions.

    2.   Accredited Construction Details

Part L1A refers to Accredited Construction Details (ACD), these can be found on the Planning Portal website (http://www.planningportal.gov.uk/england/professionals/en/1115314255826.html).

The details refer to insulation as well as air permeability and are divided into construction types:

• Timber frame
• Steel frame
• Masonry (cavity insulation)
• Masonry (internal insulation)
• Masonry (external insulation)

They include details for the important elements of the dwelling, including:

• Ground floor to wall
• Wall to ceiling
• Party walls and floors to external walls
• Eaves
• Windows and Doors

The details take the form of a site checklist.

Adoption of these details will need to be verified by Building Control or the Approved Inspector.

Sampling for Dwellings

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