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When is an extension not an extension?

At the time I suggested a guide on residential extensions could ‘wait for another time’… since then we’ve had quite a surge of requests for assessments, so I guess ‘another time’ should be now!

First off, we need to ascertain whether you need to worry about Part L at all, as some home improvements don’t need to comply. Are you building an extension, a porch or a conservatory?

If you’re building a porch or conservatory, you could be exempt from Building Regs, but how do you tell? There are four rules to follow: A conservatory or porch must be on ground level, it must be less than 30sqm, it must be thermally separate from the house (in other words, there must be a door between the house and extension) and it must not be plumbed into the main heating system.

If you definitely have an extension on your hands, you next need to check the amount of glazing, as too much north facing glass could make the house cooler, and lead to increased heating bills.

If the area of glazing in your extension is less than 25% of the new floor area, you can usually satisfy Part L by confirming the insulation levels (U-Values) in your extension meet or are lower than the following (Based on Part L 2010):

Ground floor: 0.22. External wall: 0.28. Roof areas: 0.18 where insulation slopes, 0.16 in all other areas. New windows: 1.60. New external doors: 1.80.

For example, if your new extension was 40 sqm and the total area of windows, rooflights and external doors added up to 9.9 sqm, the U-Value check would usually be fine for Building Control, as the floor to glazing ratio is 9.9 ÷ 40 = 24.8%.

If the amount of glazing is over this 25% threshold, you need to prove the additional glass will not have a negative effect on the fuel consumption of the home… and this is where we come in!

We complete two versions of a SAP assessment to prove your extension will perform better than an emission target. Contrary to what you may think, this is not a ‘before the extension’ and ‘after the extension’ comparison… it’s a bit more confusing than that.

One version of the SAP is based on the ‘proposed’ extension complete with your specification, which also includes details of the original building. As you’re unlikely to know the level of insulation in the existing structure, we have a catalogue of assumed figures that we’re allowed to use based on the construction and age of the dwelling.

Next, we create a ‘notional’ calculation. This is based on the same details as before, but we use the maximum U-Values allowed for the extension (according to Part L), we reset the glazing area of the extension to 25%, and we adjust any improvements you are making to the existing house as part of the project (for example, replacing the heating system).

The SAP assessment calculates the total expected carbon emissions for both versions. If the ‘notional’ comes out better than the ‘proposed’, it means you’ve got an issue with either poor solar gains or your insulation levels are not high enough. We’d then look for cost-effective solutions to reduce the emission rate.

If you have an extension project underway, make sure you’ve got the right bits of paperwork. If you think you need a SAP Extension assessment, get in touch before you start work on site!  

Jon Ponting

Author: Jon Ponting

This article was published by Jon Ponting on 31.08.2012.