SAP Calculations for Extensions

We produce SAP Calculations for extensions that need them. Where a full SAP isn't required, we'll get you compliant with a simpler, cheaper check. Elmhurst-accredited and accepted by Building Control nationwide.

Full SAP Calculations for glazed and larger extensions
Simpler Reference Method & Area Weighted checks where a full SAP isn't needed
We confirm the cheapest compliant route for your project
Fixed prices, revisions included

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Prefer to talk? Call us on 0333 567 9010.

Do You Need a SAP Calculation for Your Extension?

We produce SAP Calculations for extensions across the UK, but not every extension needs a full one. A whole-dwelling SAP is usually only required when an extension is heavily glazed, unusually large, or Building Control ask for a full heat-loss assessment.

Most extensions simply need to meet the building regs U-values for walls, roofs, floors and glazing to avoid a full SAP.

For most extensions, a simpler route satisfies Part L: a Reference Method check for standard glazing, or an Area Weighted U-Value calculation where openings exceed the full allowance of 25% of the floor area plus existing windows and doors covered by the extension. Both are faster and cheaper than a full SAP, and accepted by every Building Control body.

Whichever route your project needs, we can do it. If you're not sure, we'll tell you the cheapest way to comply before you commit. Check your glazing free or send us your drawings for a quote.

Three Routes to Part L Compliance

Every extension takes one of these three routes. We'll confirm which applies to yours, always starting with the simplest and cheapest.

Area Weighted U-Value

For extensions exceeding 25% of floor area plus covered existing windows and doors.

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Whole House SAP Calculation

When simpler methods can't demonstrate compliance. Same pricing as a new build.

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How It Works

1
Book or Request a Quote

Book a Reference Method or Area Weighted check online, or send us your drawings for a quote if you're not sure which you need.
2
Send Your Drawings

Upload your extension plans and elevations via the client portal. For most checks that's all we need.
3
Compliance Report

We run the calculation and send a report for Building Control, with clear advice on any changes if it doesn't pass first time.

What's Included

Every extension compliance service includes:

A clear pass, or exactly what needs to change if it doesn't pass first time
Reasonable revisions included as your design develops
Reports formatted for Building Control submission
Your compliance check credited against a full SAP if you later need one
Cost-effective advice that won't over-engineer your extension
Email and phone support throughout

Frequently Asked Questions

Do all extensions need a SAP Calculation?

No. Many extensions comply through a simpler Reference Method or Area Weighted U-Value check, which are cheaper and faster than a full SAP. A full Whole House SAP is usually only needed for heavily glazed or larger extensions, or when Building Control ask for one.

How do I know which route my extension needs?

Start with the full opening allowance: 25% of the extension floor area plus existing windows and doors covered or removed because of it. Within that allowance and with a suitable specification, the Reference Method may be enough. If openings exceed it, an Area Weighted U-value check may demonstrate compliance. If neither can demonstrate compliance, a full Whole House SAP is the fallback. Send us your drawings and we'll confirm.

Can I avoid a full SAP Calculation?

Often, yes. Most extensions pass with a Reference Method or Area Weighted check rather than a full SAP. Our free glazing calculator is the quickest way to see which applies to you.

Do conservatories need a SAP Calculation?

Usually not, if the conservatory is thermally separated from the house, with external-quality doors between the two and its own heating. If it's open-plan to the house or shares the main heating, it's treated as an extension and the same rules apply.

Do small or single-storey extensions need SAP?

Rarely a full SAP. Standard single-storey rear and side extensions with typical glazing almost always comply via the Reference Method. It's larger or heavily glazed extensions that may need more.

How much does a SAP Calculation for an extension cost?

A Reference Method check is From £95, an Area Weighted U-Value calculation is £125, and a full Whole House SAP starts from £225. If you pay for a check and later need a full SAP, we credit it against the cost.

How long does it take?

Compliance checks are typically turned around in a few working days once we have your drawings. A full Whole House SAP takes a little longer. Let us know your deadline and we'll work to it.

Will Building Control accept your reports?

Yes. All work is completed by an Elmhurst-accredited assessor using approved software, and our reports are accepted by local authority and private Building Control bodies across the UK.

Get Your Extension Compliant

Send us your drawings and we'll confirm the simplest, cheapest route to Part L compliance, and quote you a fixed price.

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