Part L of the building regulations deals with the thermal performance, or limiting the heat losses from any given building. Here, I am exploring the recent changes that affect extensions and work to existing buildings.

The legal requirement is governed by a brief few paragraphs, however there is also a suite of supportive documents called ‘The Approved Documents”. These are descriptive methods of complying with those paragraphs, and for all intents and purposes can be considered ‘the regulations’. (There can be times when methods need to step outside of these approved documents and then this distinction is more important).

Part L has undergone the most change over the last decade or so compared to the other documents as the thermal performance of buildings is a key area for improvement. (Stair safety design hasn’t changed as much for example). Recent changes until this year have solely dealt with new buildings, now extensions to existing dwellings also get a jump up in thermal performance. This is great news as improvements are desperately needed. The minimal requirements have taken a big jump up, but alas no enforced ‘consequential improvements’ have been included.

Consequential improvements already forms part of work to commercial buildings, and basically requires a percentage of the overall budget to improve the existing building as well, not just the extension. On most projects this occurs anyway as energy improvements can come from boiler upgrades, lighting etc as well as insulation, so in my mind would have been a useful tool in improving existing building stock.

Brick wall insulation

The accepted method of measuring heat losses through part of a building ‘an element’ is measured in a U-value. It is an expression of heat loss through a part of a building (relevant to the temperature difference). Measured in W/m²K. The lower the number the less heat is lost. For example a typical window u-value might be 1.6W/m²K, whereas a wall in an older extension might be 0.28W/m²K, with much more heat lost through the window.

For new dwellings or higher standards such as PassiveHaus then the whole system is just, if not more, important than the individual elements. Think of a really warm jacket with terrible stitching and gaps – you lose too much to the gaps than you save by increasing the thickness of the warm bits. On extension work calculating and measuring the whole building+extension is generally regarded as too difficult / onerous as the existing building will have too great an influence. On really good designs or exemplar projects the whole house will be considered and measured so the best approach can be taken.

On most projects therefore as long as the extension meets the minimum criteria, it is deemed to pass building regulations. It is these minimums that have jumped up in requirements this year.

The amount of glazing and orientation will also influence the design as if there is a lot of glazing, then the insulation levels in the remaining elements may have to be better still to make up for it. (Calculated as an area-weighted figure).

For reference as of July 2022, (remember the lower the number the better), all in W/m²K:

  • Roofs have gone from 0.18/0.16 depending on type to 0.15 for all roofs
  • Walls have gone from 0.28 to 0.18
  • Floors have gone from 0.22 to 0.18
  • Windows have gone from 1.6 to 1.4 with glazed doors also going from 1.8 to 1.4
  • Rooflights are 2.20

These will have an impact on previously ‘normal constructions’ For example a basic brick lean-to extension could have had the following construction thicknesses pre July 2022:

Roof: Artificial slates, battens, breather membrane with 120mm rigid insulation between and 25mm under the rafters of 150mm deep.  Previously compliant at 0.18W/m²K.

Now the same construction would require 50mm insulation below the rafters rather than 25mm to achieve 0.15W/m²K.

Brick cavity wall: Brick outer leaf, 50mm clear cavity, 50mm rigid insulation, 100mm standard grade block, wet plaster finish. Previously compliant at 0.26W/m²K.

Now the same construction would require a minimum 90mm of the same insulation, therefore installed in a min 140mm cavity width.

Clearly there are other ways to achieve the requirements, switching to insulated timber frame, using thermal blocks, fully-filling the cavity with insulation (Not recommended in exposed sites and generally TSA doesn’t specify for this reason) but the time for 300mm overall thickness cavity walls is over. A rendered blockwork wall can be compliant if a 125mm cavity with 75mm insulation is used as the blockwork performs a little better than brickwork.

The increases of 25-50mm here and there may seem trivial on spacious sites, but the industry will need to take note to avoid being caught out in the details and costs. (For example longer wall ties will be needed, window sills, insulation thicknesses, concrete foundations will all also have knock-on increases.)