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A crash course in Part L1B

Here’s a quick guide to the differences between Part L for a new building (known as L1A) and Part L for an existing building (known as L1B). This guide is equipped with the caveat that interpretations of L1B can vary between building control departments!

Limits on U-Values are different depending on whether you’re building a new wall (or roof or floor), whether you’re extending a current wall, or whether you’re renovating an existing wall.

For example, an extension wall U-value must be no higher than 0.28, but the wall of a new house can be 0.3… A renovated ground floor can be as high as 0.25, but the floor of an extension must be 0.22 or below.

If you’re renovating a building with single glazing or very old double glazing, you’d be expected to replace with new windows at a U-Value of 1.6 or better… Newly built dwellings are allowed slightly worse windows (maximum value of 2.0).

There are a couple of exemptions that can come into play… if renovating an existing wall or floor would cause complications with head height, or significantly reduce the floor area of a room, or may be let off with a reduced target.

If you are renovating an old building, it’s likely you’d be expected to insulate the original walls, floors and roofs; unless you can argue that this work is not technically or viably feasible. Extra leniency is given for listed buildings or buildings of historical importance… particularly if the building work would change the appearance of the house.

One of the most well-known aspects of Part L – the Dwelling Emission Rate – very rarely applies, as it wouldn’t be fair to compare a 200 year old barn conversion project against current regulations… also, the air leakage test is rarely requested. We’d only expect these requests from your council if you were pretty much knocking the whole building down and starting again.

If you are replacing any heating or lighting systems, they need to meet current efficiency requirements. For a gas boiler, that’s an efficiency of 88%, and for lighting you need to make sure at least 75% of fitted bulbs are low-energy.

On extensions with lots of glazing, you may be asked for a SAP to show the high levels of windows do not mean your house will be using more energy than a notional calculation… we tend to only see problems here with extensions which have large levels of north glazing, as there is not much ‘solar gain’ (heat from the sun) which means your boiler may have to work harder, and you may be using lights more than before.

Remember, a heavily glazed extension may not the same as a conservatory… but I think that difference can wait for another blog!

Finally for now, Energy Performance Certificates are not required for residential extensions, but if you already have an EPC on the house, it will need to be redone before the building is sold or rented out, as the significant changes being made are likely to affect the EPC rating.

If you’re working under Part L1B, and confused about any of these points, or others that haven’t been covered here, call us on 08458 386 387 and we’ll see if we can clear things up!

Jon Ponting

Author: Jon Ponting

This article was published by Jon Ponting on 26.07.2012.