General

10.1 This section provides guidance for new elements in existing dwellings, including all the following types of work.

a. Providing a new thermal element in an existing dwelling – follow paragraph 10.1.

b. Providing a replacement thermal element in an existing dwelling – follow paragraph 10.2.

c. Replacing windows, doors or rooflights (controlled fittings) in an existing dwelling – follow paragraphs 10.3 to 10.6.

d. Extending an existing dwelling – follow paragraphs 10.7 to 10.11.

e. Adding a conservatory or porch to an existing dwelling – follow paragraphs 10.12 and 10.13.

NOTE: Guidance for renovating elements in existing dwellings is given in Section 11.

New and replacement fabric elements

10.2 The minimum standards in paragraphs 4.7 and 4.8 and Table 4.2 should be met for both of the following.

a. New thermal elements installed in an existing dwelling.

b. Thermal elements constructed to replace existing thermal elements.

10.3 For new and replacement windows, roof windows, rooflights and doors (controlled fittings), if the entire unit of that fitting is provided, all the following apply.

a. Units should be draught-proofed.

b. Units should meet the minimum standards given in Table 4.2.

c. Insulated cavity closers should be installed where appropriate.

10.4 Building control bodies may accept, as evidence of compliance with the standards given in Table 4.2, a Window Energy Rating and/or Doorset 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 is enlarged or a new one is created, either of the following should be met.

a. The area of windows, roof windows, rooflights and doors should not exceed 25% of the total floor area of the dwelling.

b. If the area of windows, roof windows, rooflights and doors exceeds 25% of the total floor area of the dwelling, compensating measures should be taken to improve the energy efficiency of the dwelling.

10.6 The term controlled fitting refers to the entire unit of a window, roof window, rooflight or door, including the frame. Replacing glazing, or a window or door in its existing frame is not providing a controlled fitting. Such work does not need to meet the energy efficiency requirements.

Extension of a dwelling

10.7 When a dwelling is extended, elements should satisfy all of the following.

a. New thermal elements should meet the standards in Table 4.2 and paragraph 4.7.

b. Replacement thermal elements should meet the standards in Table 4.2 and paragraph 4.8.

c. New windows, roof windows, rooflights and doors should meet the standards in Table 4.2.

d. The total area of windows, roof windows, rooflights and doors in extensions should not exceed the sum of the following.

i. 25% of the floor area of the extension.

ii. The total area of any windows and doors which no longer exist or are no longer exposed due to the extension.

e. Existing fabric elements that will become thermal elements should meet the limiting standards in Table 4.3 by following the guidance in paragraphs 11.2 to 11.4.

10.8 When a dwelling 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 As an alternative approach to paragraph 10.7, 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 10.7.

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.

10.10 As an alternative approach to paragraphs 10.7 or 10.9, the Standard Assessment Procedure may be used to show that the dwelling primary energy rate, the dwelling emission rate and the dwelling fabric energy efficiency rate for the dwelling and proposed extension do not exceed those for the dwelling plus a notional extension. The notional extension should be the same size and shape as the proposed extension and comply with paragraph 10.7. The openings in the notional extension should conform with paragraph 10.7d, with the door area set as equal to the door area of the proposed extension and the remainder of the openings being classified as windows.

NOTE: Where the performance of elements of the existing dwelling is unknown, data in the Standard Assessment Procedure Appendix S should be used to estimate the performance.

10.11 When extending an existing dwelling with a total useful floor area of over 1000m2, consequential improvements may be required. Guidance in Section 12 should be followed.

Conservatories and porches

10.12 A conservatory or porch must have thermal separation from the existing dwelling. If the thermal separation is removed or the dwelling’s heating system is extended into the conservatory or porch, the conservatory or porch should be treated as an extension and paragraphs 10.7 to 10.11 should be followed.

10.13 If the conservatory or porch is not exempt from the energy efficiency requirements (see paragraph 0.14), all the following elements should meet the minimum standards of paragraphs 4.7 and 4.8 and Table 4.2.

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 10.7d 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 dwelling.

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.

Section 10: New elements in existing dwellings, including extensions