How Can I Improve My SAP Calculations?

If you are designing a new home or managing a build, you might be asking yourself: how can I improve my SAP calculations?

Getting your SAP assessment right does not have to be stressful. But too often, people leave their sap calcs until the roof is already on and the walls are plastered. At that point, fixing a failing score means panic on site, expensive redesign fees, or scrambling for last-minute fixes.

Under Approved Document L, passing your SAP assessment is all about early planning. When you sort your sap calcs at the design stage (RIBA Stage 3) before pouring concrete, compliance gaps can be identified and corrected before commitments are locked in.

Here is our practical, field-tested guide to improving your score, aligning specifications with Approved Document L standards, and saving real money on your build.

1. How Your SAP Assessment Scores Your Building

Before you change any specifications, it helps to understand what the calculation is actually looking at.

Under the SAP 10.2 rules, your dwelling is measured against three mandatory primary targets set out in Approved Document L Volume 1:

Target Emission Rate (TER): The maximum carbon emissions your property is allowed to produce per square metre, measured in kgCO2/(m²·year). Your actual score is your Dwelling Emission Rate (DER). To pass, your DER must be lower than or equal to the TER.
Target Fabric Energy Efficiency (TFEE): How much energy your building fabric needs for space heating and cooling, measured in kWh/(m²·year). Your actual score is your Dwelling Fabric Energy Efficiency (DFEE). To pass, your DFEE must beat or match the TFEE.
Target Primary Energy Rate (TPER): The maximum primary energy consumption permitted for heating, cooling, hot water, and lighting, measured in kWh_PE/(m²·year). Your actual score is the Dwelling Primary Energy Rate (DPER). To comply, your DPER must not exceed the TPER.

Under current building regulations, you cannot simply throw cheap heating into a leaky house and pass. You must also satisfy the limiting fabric backstop values in Table 4.1 (such as wall U-values of 0.26 W/m²K and roof U-values of 0.16 W/m²K), although the Notional Dwelling specification targets in Table 1.1 are significantly stricter.

2. Five Big Building Changes That Move the Needle

When our energy assessors look at a borderline SAP assessment, these are the five key areas where small tweaks make the biggest difference:

A. Fabric Insulation and U-values

Your building fabric is your first line of defence. The lower your U-value, the less heat escapes:

External walls: Aim for a U-value between 0.15 and 0.18 W/m²K, matching the Part L 2021 Notional Dwelling target of 0.18 W/m²K.
Ground floors: Aim for 0.11 to 0.13 W/m²K, using at least 150mm of PIR insulation to match the Notional specification of 0.13 W/m²K.
Roofs: Aim for 0.11 W/m²K, which typically means cross-layering 400mm of mineral wool quilt at ceiling level to satisfy the Notional target.

Investing in good insulation pays you back for the lifetime of the building without mechanical maintenance.

B. Windows, Doors, and BFRC Certification

Glazing has a big impact on your sap calcs. To get the best results:

Look for windows certified by the British Fenestration Rating Council (BFRC) with an overall window U-value of 1.2 W/m²K or better, well ahead of the 1.6 W/m²K limiting backstop.
Pay attention to the solar factor (g-value). You want a balance between 0.50 and 0.60 so you get useful warmth from the winter sun without causing summer overheating under Approved Document O.
Make sure external entrance doors are insulated composite doors rated at or below 1.0 W/m²K for opaque doors, or 1.2 W/m²K for semi-glazed units.

C. Low-Carbon Heating: Why Heat Pumps Win

Heating is where many projects run into trouble:

Air Source Heat Pumps (ASHP): Under SAP 10, the carbon factor for grid electricity dropped to 0.136 kgCO2/kWh. Because heat pumps deliver 3 to 4 units of heat for every 1 unit of electricity used (even when it is freezing outside down to -15°C), an air source heat pump makes part l compliance much easier.
Avoid electric panel heaters: Direct electric resistance panel heaters are heavily penalised in SAP because they run at a 1:1 efficiency. Unless you have massive solar arrays, panel heaters will often cause your SAP assessment to fail.
Gas boilers: If you are fitting a gas boiler, you will usually need Waste Water Heat Recovery (WWHR), a solar PV array, or balanced Mechanical Ventilation with Heat Recovery (MVHR) to compensate.

D. Adding Solar PV Panels

Solar Photovoltaic (PV) panels generate clean electricity right on your roof:

A modest, unshaded 1.5 to 2.5 kWp solar array directly offsets the building regulated energy.
On average in the UK, a standard domestic solar array saves between 1.0 and 1.6 tonnes of carbon every single year.
If your score is tight on carbon, adding solar PV is one of the quickest ways to bring your DER well below the TER.

E. 100% Low-Energy LED Lighting

Every single light fitting inside and outside the home should be low-energy LED:

Specify fittings with an efficacy of at least 75 to 80 lumens per circuit-watt.
LEDs produce bright light with very little heat, keeping your electricity demand low and assisting your SAP scores.

3. Cracking the Thermal Bridging Code (Psi-Values)

Here is something that many builders and architects miss: thermal bridging can make or break your sap calcs.

Thermal bridges are the junctions where different building elements meet, like where a wall meets a floor, a window sill, or a roof eaves. Heat loves to escape through these corners.

The default penalty: If you do not provide calculated details for your junctions, your SAP assessor has to apply default unaccredited values. This default penalty can add up to 15% to 30% of your total building heat loss (Y-value penalty of 0.20 W/m²K). That penalty alone can push an otherwise well-insulated home into a fail.
The fix: Use approved construction details, such as LABC Construction Details or manufacturer-certified junction models. Having your assessor calculate specific Psi-values drastically cuts calculated heat loss without adding construction cost.

4. The On-Site Air Tightness Checklist for Test Day

You can design the perfect house on paper, but your as-built SAP calculation depends on the physical test results achieved on site.

Under Part L 2021, sample testing is no longer permitted; every newly constructed dwelling must undergo an individual air test against the limiting standard of 8.0 m³/(h·m²) at 50 Pa and the Notional target of 5.0 m³/(h·m²). If the house leaks air, your as-built SAP will fail.

If your design specifies an air permeability score of 3.0 to 4.0 m³/(h·m²), you must hit that number on test day.

Before your ATSPACE engineer arrives with the blower door fan, walk the property and check these 12 critical leakage spots:

Plumbing waste traps: Make sure every trap under sinks, baths, and showers is full of water. A dry trap pulls air straight from the drainage stack and will fail your test.
Loft hatches: Check that the hatch is draught-sealed and latched firmly shut. Loose hatches are the number one cause of air test fails.
Soil pipe penetrations: Ensure expanding foam and mastic seal all gaps around soil pipes passing through ceilings and floors before boxing them in.
Behind bathroom and kitchen fittings: Seal behind bath panels, shower trays, and kitchen units before the units are fitted.
Window and door reveals: Check seals around frames and make sure trickle vents are closed. (Our engineers will temporarily tape trickle vents and mechanical extract fans in line with ATTMA testing standards).
Electrical backboxes: Seal cable entries through plasterboard into external cavity walls.
Service entry points: Seal around incoming water, gas, and electric ducting.
Skirting boards: Seal the junction between the plasterboard and the floor slab or deck before fitting skirting.
Extractor fan openings: Ensure ducting through external walls is sealed around the core hole.
Chimneys and flues: Ensure flues have dampers closed or are sealed for the test.
Floor penetrations: Check heating pipes where they enter through timber or screeded floors.
Site power: Make sure you have steady 240V or 110V power available and internal doors can stay open during the test.

5. BREL Photographic Evidence Required Under Part L Appendix B

A passing as-built SAP calculation now requires physical verification under Appendix B of Approved Document L Volume 1. To receive Building Control sign-off, the builder must provide geo-located, timestamped digital photographs confirming that the construction matches the design-stage SAP inputs. These photos must be captured during construction prior to closing up or plastering:

Foundations and floor perimeter: Continuity of insulation around floor edges, screeds, and thermal break details.
External walls and openings: Full cavity insulation fit, cavity closers, structural lintel insulation, and party wall edge seals.
Roofs: Unbroken insulation continuity at eaves, wall-plate junctions, gable ends, and loft hatches.
Windows and doors: Placement relative to cavity insulation, thermal reveals, and perimeter airtightness taping.
Airtightness barriers and services: Membrane overlaps, pipework penetrations, service plant data plates, cylinder insulation, and heating pipe lagging.

The completed photos are cross-referenced against the final Building Regulations England Part L (BREL) report, signed by both the On Construction Domestic Energy Assessor (OCDEA) and the principal contractor before the completion certificate can be released.

6. Frequently Asked Questions

Can I pass SAP calculations after my build is finished?
No, not safely. You must submit your design-stage SAP calculation to Building Control before construction starts on site. If you wait until the building is finished, you cannot easily change your insulation depth, window specs, or floor build-up without major structural costs.

What is the easiest way to improve a failing SAP score?
The quickest wins are switching to an Air Source Heat Pump, calculating junction-specific Psi-values using LABC details rather than taking the default penalty, and adding a 1.5 kWp solar PV array to your roof.

Does getting an airtight building mean poor ventilation?
No. You want to build tight and ventilate right. An airtight home prevents uncontrolled draughts, while continuous mechanical extract fans or an MVHR system ensure fresh, filtered air flows through every room without wasting heat.

Achieving Part L Compliance at Design Stage

At ATSPACE, we have been helping architects, housebuilders, and self-builders achieve full building compliance since 2008. With over 12,000 successful projects and more than 1,000 5-star reviews across England and Wales, our dedicated account managers make compliance straightforward.

Need a same-day quote or want our energy assessors to look over your preliminary drawings?

Phone: 0345 6465 454
Same-Day Quotes: Provided for quote requests received between 8am and 4pm, Monday to Friday
Assessment & Certification Timelines: Design-stage and as-built SAP calculations issued within 5 to 10 working days; next-day compliance certification following completed site testing