Logo

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

Get a Quote

Prefer to talk? Give us a call on 0333 567 9010 and we'll be happy to help.

Book Online

Volume discounts available for multiple units

  • Elmhurst Accredited
  • Fully Insured
  • 100s Projects Delivered
  • FHS Ready

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 glazing is over 25% of the floor area. 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.

Reference Method Check

The standard route for extensions with glazing under 25% of floor area. A quick check of your U-values against Building Regulations reference values.

Learn more →

Area Weighted U-Value

For extensions with glazing over 25%, such as large bi-folds, picture windows or roof lanterns. Balances the extra glazing against better-performing walls, roof and floor.

Learn more →

Whole House SAP

The full SAP Calculation, used when the simpler checks can't demonstrate compliance. Assesses your entire home, not just the extension.

Learn more →

Not Sure Which Route You Need?

The quickest way to find out is our free glazing calculator. Enter your floor area and glazing and it tells you whether you're within the 25% threshold.

Under 25%, you'll usually only need a Reference Method check. Over 25%, an Area Weighted U-Value calculation normally does the job. Either way, send us your floor plans and elevations and we'll confirm the cheapest route that passes. No full SAP unless it's genuinely needed.

How Much Does an Extension SAP Calculation Cost?

The right route depends on your extension, and so does the price:

  • A Reference Method check is £95.
  • An Area Weighted U-Value calculation is £125.
  • A full Whole House SAP starts from £275.

If you pay for a compliance check and later find you need a full SAP, we credit the check against it, so you never pay twice. See full details on our pricing page or get a fixed-price quote.

Pricing

Fixed prices. Full payment at the time of booking.

Reference Method Check

£95

Standard compliance check for most extensions.

Book Online

Area Weighted U-Value

£125

For extensions with glazing over 25% of floor area.

Book Online

Whole House SAP Calculation

From £275

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

Book Online

As a non-VAT-registered business, SAPgen does not charge VAT. All prices shown are the total payable.

How It Works

  1. 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. 2

    Send Your Drawings

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

  3. 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?
It mainly comes down to glazing. Under 25% of floor area, a Reference Method check is usually enough. Over 25%, you'll likely need an Area Weighted U-Value calculation. 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 £95, an Area Weighted U-Value calculation is £125, and a full Whole House SAP starts from £275. 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.

Get a Quote Check Your Glazing Free