Extending A Dwelling
Where the intention is to extend an existing dwelling the proposed works may fall under the requirements of Part L1b of the Building Regulations relating to the conservation of fuel and power.
The works will normally need to comply with Approved Document L1b (2006 edition) that sets out guidance on what is considered to be reasonable provision for meeting the Requirements for the conservation of fuel and power.
There are 3 no. different general approaches that can be used in order to obtain compliance. In all cases it is well worth liaising with the local building control body to ensure that they are happy with the proposed method of demonstrating compliance and it is recommended that you employ the services of a suitably qualified SAP rating assessor to assist with technical guidance. Contact us for further information.
1) APPROACH ONE
a) Limit the area of windows, roof windows and doors in extensions so that it does not exceed 25% of the floor area of the extension PLUS the area of any windows or doors which, as a result of the extension works, no longer exist or are no longer exposed,
and
b) Ensure that the thermal transmittance rate ('U' value) of elements of the building fabric meet certain limitations. For example, the 'U' value of a window should not exceed 1.8 W/m2K, and the 'U' value for a wall should not exceed 0.30 W/m2K.
and
c) Ensure that the heating and lighting in the extension meet certain requirements.
2) APPROACH TWO
Where more flexibility is required with regards to the size of windows, roof windows and doors in extensions; together with 'U' values, then this second approach may be used.
a) Demonstration, by calculation, that the area weighted 'U' value of all of the elements in the extension is no greater than that of an extension of the same size and shape that complies with the 'U' value standards and the 25% ruling for opening sizes outlined above.
b) Ensure that the thermal transmittance rate ('U' value) of elements of the building fabric meet certain limitations.
and
c) Ensure that the heating and lighting in the extension meet certain requirements.
3) APPROACH THREE
Approach three provides the greatest flexibility and is normally very useful in achieving compliance where an extension has a large area of glazing that exceeds the 25% rule above.
a) Undertake a SAP 2005 calculation to show that the calculated carbon dioxide emission rate from the dwelling with its proposed extension is no greater than for the dwelling plus a notional extension built to L1b standards.
b) If necessary, improvements may be made to the existing dwelling in order to achieve the required carbon dioxide emission rate. If this is the case then the improvements must meet standards laid out in the approved document. For example it may be necessary to improve the insulation to a flat ceiling under a pitched roof, in which case the 'U' value would need to achieve 0.16 W/m2K.
c) Ensure that the thermal transmittance rate ('U' value) of elements of the building fabric meet certain limitations.
and
d) Ensure that the heating and lighting in the extension meet certain requirements.
This approach requires a full SAP calculation and data will be required as per SAP Data Requirements. Where this information is unavailable then some data items can usually be inferred based upon the date of construction of the existing dwelling.