Section 10: New elements in existing buildings, including extensions
General
10.1 This section provides guidance for new elements in existing buildings, including all the following types of work.
a. Providing a new thermal element in an existing building – follow paragraph 10.2.
b. Providing a replacement thermal element in an existing building – follow paragraph 10.2.
c. Replacing windows, doors or rooflights (controlled fittings) in an existing building – follow paragraphs 10.3 to 10.5.
d. Extending an existing building – follow paragraphs 10.6 to 10.11.
e. Adding a conservatory or porch to an existing building – follow paragraphs 10.12 and 10.13.
NOTE: Guidance for renovating or retaining elements in existing buildings is given in Section 11.
New and replacement thermal elements
10.2 The minimum standards in paragraphs 4.5 and 4.6 and Table 4.1 should be met for both of the following.
a. New thermal elements installed in an existing building.
b. Thermal elements constructed to replace existing thermal elements.
New and replacement windows, roof windows, rooflights and doors (controlled fittings)
10.3 If the entire unit of windows, roof windows, rooflights or doors is replaced, all the following apply.
a. Units should be draught-proofed.
b. Units should meet the minimum standards in Table 4.1.
c. Insulated cavity closers should be installed where appropriate.
10.4 For windows used in buildings similar to dwellings, building control bodies may accept, as evidence of compliance with the standards given in Table 4.1, a Window Energy Rating from a certification scheme that provides a quality assured process and supporting audit trail from calculating the performance of the window through to the window being installed.
10.5 If a window, pedestrian door or rooflight is enlarged or a new one created, either of the following should be met.
a. The area of windows, roof windows, rooflights and pedestrian doors should not exceed the relevant percentage from Table 10.1.
b. If the area of windows, roof windows, rooflights and pedestrian doors exceeds the relevant percentage from Table 10.1, compensating measures should be taken to improve the energy efficiency of the building.
Extension of buildings other than dwellings
10.6 Constructing an extension in buildings with a total useful floor area greater than 1000m2 triggers the requirement for consequential improvements. Section 12 should be followed.
10.7 An extension should be regarded as a new building, and guidance in Sections 1 to 9 should be followed, if the proposed extension has a total useful floor area that is both of the following.
a. Greater than 100m2.
b. Greater than 25% of the total useful floor area of the existing building.
If the proposed extension does not meet criteria (a) or (b) above, the guidance in paragraphs 10.8 to 10.11 should be met.
10.8 When a building is extended, any fixed building services or on-site electricity generation that are provided or extended should comply with the guidance in Sections 5 and 6.
10.9 When a building is extended, elements should satisfy all of the following.
a. New thermal elements should meet the standards in Table 4.1.
b. Replacement thermal elements should meet the standards in Table 4.1.
c. New windows, roof windows, rooflights and doors (controlled fittings) should meet the standards in Table 4.1.
d. Existing fabric elements that will become thermal elements should meet the limiting standards in Table 4.2 by following the guidance in paragraphs 11.2 to 11.4.
In addition, the area of openings in the extension should not exceed that given in Table 10.1, if either of the following areas are greater than that of the existing building.
a. Window and pedestrian doors as a percentage of exposed wall.
b. Rooflights as a percentage of area of roof.
Table 10.1 Maximum area of openings in the extension
10.10 As an alternative approach to paragraph 10.9, the area-weighted U-value of all thermal elements in the extension should be shown to not exceed the area-weighted U-value of an extension of the same size and shape that complies with paragraph 10.9. This includes the opening area standards in Table 10.1.
The area-weighted U-value is given by the following expression.
[(U1 × A1) + (U2 × A2) + (U3 × A3) + …]/ (A1 + A2 + A3 + …)
Where:
U1 = the U-value of element type 1
A1 = the area of element type 1
and so on.
10.11 As an alternative approach to paragraphs 10.9 or 10.10, an approved calculation tool may be used to demonstrate that the building primary energy rate and the building emission rate for the building and proposed extension do not exceed those for the building plus a notional extension. The notional extension should be the same size and shape as the proposed extension and comply with paragraph 10.9.
All calculations should include all consequential improvements that may apply.
Conservatories and porches
10.12 A conservatory or porch must have thermal separation from the existing building. If the thermal separation is removed or the existing building’s heating system is extended into the conservatory or porch, the conservatory or porch should be treated as an extension and paragraphs 10.6 to 10.11 should be followed.
10.13 If the conservatory or porch has thermal separation from the existing building, and the existing building’s heating system does not extend into it, and is not exempt from the energy efficiency requirements because of its size or another reason outlined in paragraph 0.18, all the following elements should meet the minimum standards in Table 4.1.
a. New thermal elements.
b. Replacement thermal elements.
c. New windows, roof windows, rooflights and doors. The limitations on area of windows, doors and rooflights in paragraph 10.9 do not apply.
In addition, both of the following should apply.
a. Any walls, doors and windows should be insulated and draught-proofed to at least the same extent as in the existing building.
b. Fixed building services and/or on-site electricity generation within the conservatory or porch should both:
i. meet the standards in Sections 5 and 6
ii. have independent temperature and on/off controls.
