First off, the U-Values required for building elements in an extension are slightly different to those required for a brand new building - External walls of extensions should be no higher than 0.28, floors at 0.22 and roofs should be no higher than 0.18. Also, windows or rooflights in the extension should be 1.60 or lower.
Usually the existing part of the house does not need to be upgraded at all; however Building Regs does recommend upgrading and replacement where reasonable. If any parts of the house are being upgraded or rebuilt, they should meet U-Values of 0.25 for floors, 0.30 for walls and 0.18 for roofs.
The same rule applies to changes to heating, lighting and ventilation systems. If you are upgrading any of these, you need to make sure the efficiency of the system is no worse than the figures as listed in the Domestic Building Services Compliance Guide.
Upgrading the existing building is currently only mandatory on large commercial buildings, where 10% of the budget for the build must be allocated to renovating. Talks to expand this to cover the domestic sector were scrapped in a Government U-Turn which was branded the ‘Conservatory Tax’ by the red top press.
If your new extension is defined as ‘heavily glazed’, the expected carbon emissions of the extension need to be looked at in more detail to minimize heat loss. An extension is ‘heavily glazed’ if the total area of windows in the extension is greater than 25% of the new floor area PLUS the area of any windows in the existing house which are being removed as part of the extension works.
So, if you’re building a 20sqm sun room onto the side of a house, and removing existing patio doors (2sqm in area) to make it open to the existing house, the extension would be ‘heavily glazed’ if the new area of windows and rooflights exceeds 7sqm. (25% of floor area = 5sqm, plus 2sqm glazing removed).
In cases like this, we need to calculate the expected annual CO2 of the existing house PLUS the extension, and compare the result to a notional target. Your extension will show compliance with Part L1B providing all of your U-Values meet the limitations as shown above, and providing the actual emission rate is no higher than the target.
So how do you make sure your extension shows compliance with SAP? Fortunately, the target isn’t as stringent on extensions as it is for new buildings, so providing the project has really good insulation levels, we don’t usually see any problems.
Orientation is a key factor though, so if your extension has lots of north facing windows the building won’t benefit greatly from solar gains, which means the heating system will need to work harder, and you’ll find it more difficult to comply without making upgrades.
Another common issue can be changing the heating system from gas or oil to grid electricity. This is because electricity has a much higher ‘emissions factor’ than most other fuels. Part L1B does not allow you to change a heating system if the expected production of CO2 is greater from the new system. This isn’t the same as the efficiency of the system, so if you have a concern about changing the heating system, our Part L Extension experts will be able to help.