Free Area Weighted U-value Calculator for Extensions
More glass in your extension? Check whether your insulation and glazing balance the heat loss. Enter your dimensions and U-values, get a free pass or fail result, then try changes to your design.
No sign-up or payment needed to calculate. If your design passes, a detailed PDF report for building control is £39.99.
accepting new projects
An extension heat loss check, without the spreadsheet
Bifold doors, large windows and rooflights can push an extension beyond its glazing allowance. An Area Weighted U-value calculation checks whether better insulation or glazing balances that additional heat loss.
This free online calculator builds the comparison from your measurements. It works out the exposed wall, floor and roof areas, deducts the openings and compares your proposed extension with a reference extension of the same size and shape.
Use it while planning your specification. You can change the U-values or opening sizes and calculate again before committing to products or materials.
Only need to check the 25% glazing rule? Start with our free glazing allowance calculator.
How to use the calculator
- Set out your extension. Add the project address, internal dimensions and wall connections. The visual plan helps you identify exposed and connected walls.
- Add the roof and openings. Choose the roof shape and ceiling arrangement, then enter existing openings covered by the extension and your proposed windows, doors and rooflights.
- Check the result. Enter the calculated U-values as you go. See whether the design passes, try changes or buy the PDF when you're ready.
Have these details ready
- Your project address and the extension's internal width and depth.
- The exposed parts of each wall, ceiling heights and roof shape.
- Sizes of existing windows and doors covered or removed by the extension.
- Sizes of proposed openings, including frames, and their product U-values.
- Calculated U-values for the proposed wall, roof and floor construction.
The calculator uses U-values you supply; it doesn't calculate them from insulation thicknesses. We can calculate your individual construction U-values if you need them.
Will it work for my extension?
The tool supports single storey rectangular and square extensions in England. Walls can be exposed, connected to heated rooms or partly connected. A wall adjoining an unheated garage counts as exposed.
Choose a flat roof or pitched roof with a level ceiling. For vaulted ceilings, it supports mono pitch, gable, hipped and one-end-hipped roofs, using your minimum and maximum internal ceiling heights.
Have an L-shaped extension, a more complex roof or a rooflight through a cold loft? Use our assessed Area Weighted U-value service so we can work from your drawings.
What is an Area Weighted U-value?
A U-value describes how readily heat passes through a wall, roof, floor or opening. Lower values mean less heat loss. Area weighting accounts for how much of each element you have: a large window has more influence than a small one with the same U-value.
For each element, multiply its area by its U-value. Add those figures together, then divide by the total area of the extension's exposed envelope. This gives the average U-value for the comparison.
The proposed average must be no higher than the reference average. Better insulation can help offset extra glass, but the balance depends on both the areas and the U-values.
The calculator follows paragraph 10.9 of Approved Document L, Volume 1, 2021 edition with 2023 amendments, for England.
Free to calculate. Buy the report when you're ready.
Try your design before deciding whether you need a report. The PDF uses the address, measurements and U-values you confirm at checkout.
Free calculation
Work through the visual forms, see a pass or fail result and try different opening sizes or specifications. No account or card details needed.
Start calculatingPDF report - £39.99
For a passing design, buy the detailed proposed and reference calculations, opening schedules and comparison result. Open it after payment, receive it by email and keep it in your client portal.
Your individual U-values must already be calculated. Submit the report to building control for approval before work begins or materials are ordered.
Prefer us to do the calculations?
Send us your plans and we'll handle the measurements, calculate the individual U-values and complete the Area Weighted check. You'll receive a report for building control, with advice and reasonable revisions included.
The calculator report uses your inputs. Our assessed service includes the work needed to prepare them.
Questions about the calculator
Is this a free Area Weighted U-value calculator?
Yes. Enter your project details and see whether the design passes for free. The detailed PDF report is optional and costs £39.99. You don't need to pay to try another specification.
Can I use the report for building control?
The PDF sets out the Area Weighted comparison for your extension. Check your measurements and use properly calculated construction U-values. Building control makes the approval decision, so submit it before work starts or materials are ordered.
Does a fail mean I need smaller windows?
Not necessarily. Better insulation or a lower glazing U-value may change the result. If improving the extension's fabric isn't practical, a whole house SAP comparison can also account for useful solar gains.
Is this the same as a glazing allowance calculator?
No. A glazing calculator checks how much opening area the reference method allows. This tool goes further by comparing the heat loss through the whole extension's fabric.
