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Domestic Extensions or Conversions

Domestic Conversions or Extensions under Part L1B (SAP calculations)

Part L1B relates to works to existing buildings and domestic extensions. Normally an extension is designed using elemental u-values (i.e. walls, floor, roof, windows and doors need to achieve a certain thermal performance).

Some designs include large areas of glazing far in excess of the permitted 25% limit of AD L1B. An alternative to the elemental approach is to look at the proposed extended dwelling and to show, by calculation, that by making compensatory improvements to the existing building or extension, that the proposals are no worse than a compliant extension. For example this might mean inserting loft insulation up to current standards in your existing loft space or renewing an old boiler.

Buildings covered in L1B

  • Construction of an Extension
  • A material Change of Use, or a change to the building's Energy Status, including loft and garage conversions
  • The provision or extension of Controlled Services of Controlled Fittings
  • The replacement of a Controlled Services of Controlled Fittings

If a building is part commercial, or is a number of dwellings with a common area, such as a common corridor or stairwell, any work covered by Part L1B is applicable to the flat (domestic) only, and the commercial or common areas will be subject to the requirements of Part L2B.


  • Listed Buildings
  • Conservation Area
  • Ancient Monuments
  • Buildings of a special architectural interest
  • Buildings of traditional construction with permeable fabric that both absorbs and readily allows evaporation of moisture
  • Conservatory (or porches) under 30m2 where the walls, windows and doors that separate it from the rest of the main house are retained including and the heating system of the main is not extended. Note: conservatory is defined as having a minimum of 50% of the walls glazed, and 75% of the roof glazed, otherwise it is defined as an extension.

If the energy efficiency measures proposed could have a potential detrimental effect on the character or appearance of the building (as could be the case with the first five bullets above) then this should be discussed with your Conservation Officer.

Conversions and Extensions Part L1B

Opening up the Conservatory

If a new conservatory is open to the rest of the house, or if heating from the rest of the house is extended into it, it essentially becomes a very highly glazed extension, and you will require consequential improvements to the rest of the house to compensate for the additional heat loss (each thermal element also needs to meet minimum requirements).

Change of Use

A house converted to flats, or vice versa, will not only require Building Regulations for compliance but will also need an Energy Performance Certificate (EPC).

Information required to perform calculations

  • A full set of scaled drawings (illustrating floor plans, internal sections, external elevations and an external window/door schedule). 
  • Construction specification (materials and thicknesses of each layer of construction in floor, walls and roofs).
  • Window and Door u-values (values must include the frame).
  • Details of heating and hot water system (make and model is required).

The above is the minimum requirements although if the property is a change of use or renovation then all of the above will be required for the “before” and “after” works.

We provide quality and professional advice on any type of work. All our quotes include full guidance and an end result which will comply with Building Control Officers.

Commercial New Build (SBEM)
Commercial Extensions or Conversions
Domestic New Build (SAP)
Domestic Extensions or Conversions

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