3 ways to comply with the standards for domestic extensions

Extending your home to comply with the standards

So, you’re building an extension on your house. What do you have to do to make sure it complies with the standards? This blog explores the options…

When determining compliance for an extension, there are three routes to showing that it thermally complies with standards – these are:

  • follow a package of target values laid out in the standards for domestic extensions.

To comply with this approach, the area of openings for the extension must be less than 25% of its floor area, plus a bit extra from the area of any windows or doors that will be built over as a result of the extension works.

Alternatively, you can

  • show compliance by undertaking an area weighted U-value calculation for the proposed extension and for one built to the notional values and opening allowance, trading off improved thermal elements against slightly larger amounts of glazing.

This is known as a compensatory approach and can be useful where the glazed area is only a little over the glazing allowance.

The other approach, known as  ‘design flexibility’, is to:

  • use the SAP calculation procedure to show that the total CO2 emissions from the entire extended dwelling as it will become, are no greater than if the dwelling had been improved to the standards outlined (with the notional glazing allowance). 

Upgrades to the existing dwelling, can be made to compensate for varying individual elements and greater glazing allowances.  This approach is more complicated and requires a lot more information about the existing dwelling as well as the extension proposals.

The notional values are balanced around minimising excessive losses from highly glazed extensions, as even with the best available glazing, more heat will be lost from openings than would be lost through a well insulated wall or roof.

The ‘required’ values vary across the UK, with different targets required from 2014 for England, Scotland and for Wales. (In Scotland the target differ according to the existing performance of the dwelling):


Design flexibility, or compensatory approaches will require pushing the standards further to try and balance any poorer values or greater glazing requirements.

Ideally, when contemplating an extension, we should aim for a reduction, or at least no net increase, in carbon emissions and fuel bills; but failing that, it is not unrealistic to expect new elements to achieve a new build level of performance – and there is also often opportunity to improve the existing property at the same time that extension works are undertaken.

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Jon Ducker is a qualified energy assessor working for Kingspan Insulation Ltd. He has an extensive knowledge of energy efficiency, renewable energy systems and sustainability in buildings with an expert knowledge of the relevant sections of buildings regulations and standards and their interactions with SAP. He provides authoritative advice regarding energy assessments for a wide range of public and private sector clients.

0 comments on “3 ways to comply with the standards for domestic extensions
  1. Hello, can you build a brick built extension to a kingspan built house? We were told when we bought the kingspan house that no extension or conservatory could be added to the house as drilling into the outside walls would potentially ruin the kingspan insulation panels. We would like to stay in this house and extend in a few years (garage + 2 bedrooms above at the side, and maybe a conservatory at the back through existing patio doors) but are unsure about whether this is possible.
    Thank you for the advice.

    • The answer is yes. You can provide future adaptions and alterations to a Kingspan TEK house. However you (the client/home owner) need to engage with the original engineer for the project and the structural calculations that should have been provided at the time. The engineer will then look at the existing structural loadings at the particular area they want to make the necessary alterations and recalculate what is required in terms of timber to create the opening.

      Hope this helps.

  2. The table covering the U- values in the different areas of GB is great. Could you please add Northern Ireland and the U-values required there?

  3. Hi Jon, we are proposing to build an extension on our cottage using Sinusodal insulated panels. We have been advised by our architect that the U value required by our local authority (Fife, Scotland) is 0.11 w/m2K. We intended to use the inside of the panel as the ceiling finish. Can you please advise us on requirements that will satisfy our local authority please. Thank you

  4. Hi can you tell me what U-values I should be working to for a rear extension please,
    floor, walls and pitched roof (and flat) also windows doors etc

    Thank you

  5. Hi Jon,

    When working out the floor area of the building / extension, do you also include the floor area under all external walls, or are you supposed to use the internal floor area for the calculations?

    Many thanks.

  6. Hi Jon. Could you please advise who I would contact about whether my single story extension has adequate insulation. It was built for a disabled child who can’t control her body temperature and overnight the room is very cold and she is vulnerable. It is the middle of July and last night the temperature dropped to 11 degrees. I’m wondering if there is something fundamentally wrong with the extension.

    • Hi Jules

      Thanks for your question.

      While it’s not clear from your post how well insulated the extension is (are the walls, roof and floor insulated? And if so, to what standard?), we’ve done our best to give you a little guidance.

      Is your extension heated? A well-insulated building will help to retain heat, but if it is unheated then it will slowly reduce in temperature until the inside matches that of the outside. This might be over several hours, but with no top-up heating eventually the building will cool down. It’s therefore important to ensure that the extension is heated. Perhaps consider the upper and lower ranges of temperature that the child in question considers comfortable, and then consider the building’s performance at different times of the year as well as its heating strategy. If you don’t have one, maybe look at a modern programmable learning thermostat and control system?

      If your extension is heated, it’s important to ensure that the walls, roof and floor are each well-insulated. We can help with this, although we will need to know a little more information about how each of these elements are currently insulated and how your extension is built.

      Hope this helps and that you find a solution.

  7. Hi
    When working out the floor area of the building / extension, can you also include the floor area under all external walls for calculations so more glazed area is possible?

  8. Hi, I’m an accredited SAP assessor so I thought I might clarify a couple of points.

    You will need to provide your building control officer with a SAP report if the following criteria are met:

    The extension or conservatory has glazing of more than 25% of the floor area (measured internally).

    AND it is not separated from the main dwelling by an external type door.

    The idea is to demonstrate that you have offset the additional loses created by having a highly glazed extension. To offset the loses you will need to improve the extension u-values to below the L1B maximum values or improve the main dwelling (by upgrading the boiler for example).

    Stewart Jones

    • Hi Stewart

      Thanks for taking the time to read my post.

      In the original article, the glazing allowances and means of showing compliance are noted. You don’t have to use SAP if the glazed area is only a little bigger than the allowance, you can do a simple area weighted U-value calculation. For larger glazed areas, greater design flexibility will be needed (SAP).

      It’s not a straight 25% of floor area allowance for glazing, you also get the area of glazing of the existing external wall that is covered by the extension. For example, if the extension floor area were 20m² and an existing door and window of total area of 3m² were to be covered over, then the openings allowance for the extension would be (20m² x 0.25) + 3.00m² = 8m² allowance. If the extension openings came in below this, then the simple approach can be followed, otherwise either the area-weighted average U=value approach, or the greater design flexibility approach should be used.

      The separation with an ‘external door’ is irrelevant if the extension is heated by the main heating system, or if the existing wall between main dwelling and extension is uninsulated (i.e. not thermally separated). If the two areas share heating, it’s still an extension. To classify as a conservatory, there needs to be thermal separation of walls and doors and either there needs to be no heating to the proposed conservatory, or it can be independently heated separate from the main system.

      You don’t use SAP to show compliance for conservatories.

      The greater design flexibility route using SAP (for extensions) requires production of two complete SAPs for the full existing dwelling one with the extension built to ‘Notional’ standards (using glazing allowance and the L1B simple compliance U-values and the second assessment using the ‘Actual’ proposed extension. Compliance is achieved if the actual calculation achieves the target set by the notional calculation, comparing DERs achieved for each. It is rare that full information is available for the existing property, as it requires full plans, sections, elevations for the existing property, as well as for the extension.

      Many thanks

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