Whole House SAP for Extensions

What is whole house SAP?

Complete energy model of your entire home including the extension. The most comprehensive compliance method.

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When do I need it?

When reference method and area-weighted both fail - usually heavily glazed or complex designs.

When it applies →

Why does it help?

Accounts for solar gains, heating efficiency and thermal mass - factors simpler methods ignore.

How it works →

How much does it cost?

From £200. Area-weighted fee (£95) credited if you tried that first.

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How long does it take?

2-4 working days once we have your drawings and existing house details.

See process →

What if SAP fails?

We provide ranked recommendations. Targeted upgrades usually bring designs into compliance.

Passing strategies →

Heavily glazed extension? Whole house SAP is often the only way to get approval. We'll model your design and find the path to compliance.

Get SAP Quote → View Pricing

2-4 day turnaround Elmhurst OCDEA

What Is Whole House SAP for Extensions?

Whole house SAP is a full energy assessment that models your entire home - existing property plus proposed extension - to demonstrate Part L compliance (section 10.10 of Approved Document L).

Unlike the reference method or area-weighted approach, whole house SAP considers the complete picture: how the existing building performs, how the extension integrates, how much solar energy enters through glazing, and how efficiently the heating system operates.

This comprehensive approach often approves designs that simpler methods reject. A large south-facing glazed extension, for example, captures significant solar gains that offset its heat losses. The reference method and area-weighted calculation can't account for this - whole house SAP can.

When Do You Need Whole House SAP?

Whole house SAP is the final compliance route when simpler methods don't work. Typical scenarios:

Glazing significantly exceeds 25% (often 30%+ of floor area)
Area-weighted calculation fails because insulation improvements aren't enough
Complex designs with multiple glazed elements (bifolds + roof lanterns + windows)
North-facing extensions with substantial glazing
Architect-led designs prioritising natural light and views

If you've already tried the reference method and area-weighted approach, whole house SAP is your next step. If your design is clearly heavily glazed, it may be worth skipping straight to SAP to save time and fees.

Why Whole House SAP Approves More Designs

SAP considers factors that simpler compliance methods ignore:

Solar Gains

✓ South and west facing glazing captures free heat
✓ Large sliding doors facing the garden often perform well
✓ Solar gains offset heat losses through glass
✓ Orientation matters - same glazing performs differently north vs south

Heating Efficiency

✓ Modern condensing boilers improve whole-house results
✓ Heat pumps significantly boost SAP performance
✓ Good controls and zoning help
✓ Existing efficient systems benefit the extension calculation

Existing House Performance

✓ Well-insulated existing properties can absorb more glazing
✓ Recent upgrades (loft insulation, boiler) help
✓ The extension is judged in context of the whole dwelling
✓ Good existing fabric creates headroom for glazed extension

Thermal Mass

✓ Solid floors store and release heat slowly
✓ Masonry walls buffer temperature swings
✓ Reduces overheating and smooths heating demand
✓ Benefits both SAP and Part O compliance

How the Whole House SAP Process Works

We model your complete dwelling to demonstrate compliance.

1

Send us your drawings

Extension plans plus details of the existing house - floor plans, construction details, heating system. We'll tell you if anything's missing.
2

We build the SAP model

We create a full SAP 10 model of the existing dwelling plus extension, accounting for solar gains, heating efficiency and all thermal elements.
3

Review results and recommendations

You receive a clear pass/fail result. If improvements are needed, we rank them by cost-effectiveness so you can make informed decisions.
4

Building Control reports

Once compliant, we issue your SAP report and Part L documentation for Building Control submission.

What If Whole House SAP Doesn't Pass?

Even at the SAP stage, most extensions can be brought into compliance. We provide recommendations ranked by cost-effectiveness:

Quick Wins

Small solar PV array (often cheapest fix)
Upgrade to triple glazing
Add 50mm roof insulation
Improve heating controls

Larger Improvements

Reduce largest glazed element slightly
Add MVHR ventilation system
Upgrade existing house insulation
Install heat pump (if replacing boiler anyway)

We're not here to gold-plate your extension. If a few solar panels get you over the line, that's what we recommend. All reasonable revisions are included until you pass.

Whole House SAP Pricing

Fixed pricing for extension SAP calculations. Area-weighted fee credited if you tried that route first.

Extension SAP

From £200

Full SAP model of existing house plus extension. Includes revisions until you pass.

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As-Built SAP

From £80

Final SAP update at completion. Required if you make changes during construction.

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View Full Pricing →

What We Need for Whole House SAP

To model your complete dwelling, we need details of both the extension and existing house.

Extension Details

✓ Floor plans with dimensions
✓ Elevations showing glazing
✓ Construction build-ups (walls, floor, roof)
✓ Window/door schedule with U-values

Existing House

✓ Floor plans (can be sketched if no drawings)
✓ Construction type and age
✓ Any known insulation upgrades
✓ Heating system make/model

Don't have everything? We can work from reasonable assumptions for older elements. The extension specification is most important.

Whole House SAP FAQ

Do I need details of my existing house?

Yes, but we can work with reasonable assumptions if you don't have drawings. Construction type, approximate age, and heating system are the key details. We can estimate insulation levels based on building age and type.

What if I don't know the existing house construction?

We use SAP default assumptions based on building age. A 1930s house has different defaults to a 1990s house. If you've made upgrades (loft insulation, new boiler), let us know - these improve your results.

Is the area-weighted fee credited?

Yes. If you paid £95 for an area-weighted check and it didn't pass, that's credited toward your SAP. You're not paying for two separate services.

What's the difference between design stage and as-built SAP?

Design stage SAP is based on your proposed specification - what you plan to build. As-built SAP is completed once construction finishes, using the final specification and air test result if required. Most extensions only need design stage.

Will my extension definitely pass with SAP?

Almost always. The comprehensive modelling usually finds a compliant solution. If your initial result fails, we recommend cost-effective improvements. Very heavily glazed north-facing extensions are the hardest, but even these can usually pass with targeted upgrades.

Ready to get your extension SAP?

Get SAP Quote → View SAP Pricing

All SAP calculations completed by an Elmhurst-accredited OCDEA and accepted by Building Control nationwide.

Whole House SAP for Extensions