What you need to know about Domestic Extensions

Domestic Extensions

An extension to an existing dwelling can be treated in various ways.  There are some get-outs but generally the following guidance should keep you heading in the right direction.

At its simplest, keep the U values (heat losses) of the floor, walls, roof and doors, windows and rooflights below the maximum permitted and restrict the area of the openings to 25% of the additional floor area plus the area of any doors, windows and roof lights that either no longer exist or are no longer exposed because the extension will be built.  Also, provide a suitably efficient heating system and associated controls and dedicated low energy light fittings.

Another alternative is to make sure that the area weighted U value of what you're proposing to do is no worse than an extension that complies with the simplest solution above.  This will permit small increases in the area of glass that you can have, but it does mean that the insulation levels of the floor, walls, roof and doors and windows will have to be better than they would have had to be in the first example.  If necessary, it may be possible to slightly reduce the area of openings so that this area weighted U value calculation demonstrates compliance.  This route will not cope with large amounts of glass.

The third way of demonstrating compliance is to use SAP 2012 methodology to demonstrate that the CO2 emission rate from the whole dwelling with its proposed extension is no greater than for  the whole dwelling plus an extension that complies with the simplest solution above. This gives you scope to up-grade the insulation values and/or up-grade the heating system of the existing house/ bungalow/flat.  However there is a potential downside to this in that any up-grade should be implemented to the full level of the guidance given in Approved Document L1 B whether or not it is necessary for the calculation to show compliance.  Cavity wall insulation and extra roof insulation are the most obvious ways of improving things.

If your proposed extension has lots of glass and is beginning to resemble a conservatory, (or maybe it is a conservatory) the easiest route is to keep it thermally separate from the rest of the house (ie leave external quality doors and double glazed windows in place between the dwelling and extension and have no heating).  If it is heated, compensatory measures will be required. 

There is also now a requirement for Accredited Construction Details to be used on any new build work and where appropriate on the existing. 

Approved Document L1B (2010 Edition) as revised in 2013 gives a fuller picture of the technical requirements.

If you want to discuss your proposals, please either give us a ring or send us an e mail.