Whole House SAP for Extensions
When the simpler methods can't demonstrate compliance, a Whole House SAP assesses your entire dwelling including the extension. Solar gains and heating efficiency often approve designs the simpler checks reject.
- The fallback when Reference Method and Area Weighted can't pass
- Assesses the whole dwelling, not just the extension
- Can unlock compliance for heavily glazed designs
- Area weighted fee credited towards your SAP
Prefer to talk? Give us a call on 0333 567 9010 and we'll be happy to help.
SAP Calculations
- Small Property <80m² £275
- Medium Property 81-150m² £350
- Large Property 151-250m² £425
- Extra Large Property >250m² Get a quote
Volume discounts available for multiple units
- Elmhurst Accredited
- Fully Insured
- 100s Projects Delivered
- FHS Ready
What Is a Whole House SAP for an Extension?
A Whole House SAP is the same full SAP Calculation used for a new build, applied to your existing home plus the extension. It demonstrates compliance under Section 10 of Approved Document L, Volume 1 (Dwellings) by modelling the energy performance of the whole dwelling, rather than checking the extension's elements one by one.
Because it looks at the whole picture, it can approve designs the reference method and area-weighted checks reject. A large south-facing glazed extension, for example, brings in solar gains that offset its heat losses, and the simpler methods can't account for that.
When Do You Need One?
A Whole House SAP is only needed when the simpler, cheaper routes can't demonstrate compliance. That usually means one of the following:
- Glazing well over 25% of the floor area, often 30% or more
- An area-weighted calculation that doesn't pass because better insulation alone can't compensate
- Complex designs with several glazed elements (bi-folds, roof lanterns and large windows together)
- Heavily glazed north-facing extensions with little solar gain
- Architect-led designs prioritising light and views
We always try the cheaper routes first. If your design is clearly heavily glazed, though, it can be worth going straight to a full SAP to save time.
Why a Full SAP Approves More Designs
A whole-dwelling SAP accounts for things the simpler checks ignore:
- Solar gains: south and west-facing glazing brings in free heat that offsets losses
- Heating efficiency: a modern boiler or heat pump lifts the whole-house result
- Existing house performance: a well-insulated home creates headroom for more glazing
- Thermal mass: solid floors and masonry walls smooth heating demand
- The extension is judged in the context of the whole dwelling, not in isolation
How It Works
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1
Send Your Drawings
Upload the extension plans and elevations, plus details of the existing house. We'll tell you if anything is missing.
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2
We Build the SAP Model
We create a full SAP model of the existing dwelling plus the extension, accounting for solar gains, heating and every thermal element.
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3
Results and Recommendations
You get a clear pass or fail. If changes are needed, we rank them by cost so you can decide what to do.
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4
Building Control Report
Once compliant, we issue your SAP report and Part L documentation for Building Control.
What We Need From You
To model the whole dwelling we need details of both the extension and the existing house.
The extension:
- Floor plans with dimensions
- Elevations showing the glazing
- Construction build-ups for walls, floor and roof
- Window and door schedule with U-values
The existing house:
- Floor plans (a sketch is fine if you have no drawings)
- Construction type and rough age
- Any known insulation upgrades
- Heating system make and model
Don't have everything? We can work from reasonable assumptions for older elements. The extension specification matters most.
If It Doesn't Pass First Time
Even at the SAP stage, most extensions can be brought into compliance. We recommend changes ranked by cost, starting with the cheapest:
Quick wins:
- A small solar PV array (often the cheapest fix)
- Upgrade to triple glazing
- Extra roof insulation
- Better heating controls
Larger improvements:
- Reduce the largest glazed element slightly
- Add MVHR ventilation
- Improve the existing house insulation
- Install a heat pump (if you are replacing the boiler anyway)
We won't over-engineer your extension. If a few solar panels get you over the line, that's what we'll recommend, and reasonable revisions are included until you pass.
Pricing
As a non-VAT-registered business, SAPgen does not charge VAT. All prices shown are the total payable.
Frequently Asked Questions
Do I need details of my existing house?
What if I don't know the existing construction?
Is my area-weighted fee credited?
What's the difference between design and as-built SAP?
Will my extension definitely pass?
Get Your Extension SAP
Send us your drawings and we'll model your whole home. If a simpler route would pass, we'll tell you.