Why Your Planning Condition Just Got Harder: 7 SAP Lessons Every Developer Should Know
Updated: Sep 13
Since early 2026, PassiHVAC has been undertaking SAP calculations and energy statements for private developers, with a particular focus on smaller residential schemes so far. It's a different world from the volume housebuilder end of the market: fewer standard house types to fall back on, less in-house technical resources on the client side, and often a planning department that has never seen the applicant's name before.
A handful of patterns have shown up repeatedly across these projects. None of them are exotic. All of them cost someone time or money the first time they're missed. This is what we've learned so far.
Key takeaways
Most SAP instructions arrive once a planning condition is in place and the design is largely fixed — but small, late-stage changes to facade or roof pitch can still meaningfully improve the result.
SAP is a balance of several metrics (fabric, heating emissions, fuel cost), not an insulation scorecard — fabric improvements have a ceiling.
Thermal bridging assessed with default, non-accredited values can undo fabric gains made elsewhere in the building.
The Future Homes Standard is confirmed for 24 March 2027 (24 September 2027 for higher-risk buildings); SAP and the Home Energy Model will run in parallel for a transition period rather than SAP disappearing overnight.
1. Most instructions arrive after the design is already fixed
The typical trigger for a first enquiry is a planning condition: the council wants Part L compliance demonstrated, sometimes better than the minimum, along with evidence of how CO2 emissions will be reduced. By this stage the design is largely locked in, and clients will usually only accept recommendations involving minor fabric or heating changes — not a redesign.
That's still workable in most cases. What surprises people is how much a small, late-stage change to the facade or roof pitch can move the result, mainly by reducing thermal bridging or increasing usable roof area for PV. These are the changes worth raising even at a late stage, because they tend to be cheap relative to the compliance headroom they buy.
2. Good insulation doesn't automatically mean a good score
SAP isn't a single insulation scorecard — it's several metrics that all need to be satisfied together:
Fabric energy efficiency
Primary energy
The emission factor of the heating system
Efficiency and running cost of the heating system
On-site renewables
Fabric improvements also have a ceiling. Beyond a certain point, adding more insulation stops moving the result, and the marginal gain has to come from elsewhere — usually the heating system or on-site generation. Getting a good SAP outcome is a balancing act across all the inputs, not an insulation competition.
A highly insulated building can still underperform on the metrics that matter if the heating strategy doesn't complement it.
Underneath the headline rating, the mechanism is simple to state and easy to lose sight of: Compliance comes down to these 3 conditions.
The actual emission rate (DER) sitting below the notional value (TER)
The actual fabric performance (DFEE) performing better than the notional one (TFEE)
And finally, the actual primary energy rate to run the property (DPER) lower than the notional one (TPER)
However, many councils only require demonstrating reduced emission. For instance, GLA requires minor residential projects to show DER is less than TER by 10%
3. A client-side decision late in the build can undo months of work
This is the one that stings. We worked out an optimised heating strategy for a client with no strong preference of their own, and they signed it off. Once construction was under way, an advisor who had never been part of our conversations suggested something entirely different — and it was adopted on site. The as-designed rating we'd carefully built no longer matched what was going into the ground.
The lesson isn't really about that particular advisor. It's that specification decisions on a live SAP calculation need a single point of authority, and any late change — however well-intentioned — needs to be checked against the model before it's built, not after.
Specification decisions on a live SAP calculation need a single point of authority.
This isn't a new problem, or one unique to small developments. The Zero Carbon Hub's Performance Gap work looked at exactly this failure mode across the wider industry: as-built dwellings drifting from their as-designed rating because information wasn't flowing clearly between assessor, developer, and site. The scale is different at the volume end of the market, but the root cause is the same one we keep seeing.
4. A clear information timeline saves everyone money
Early on, we assumed clients understood the Building Regulations process well enough that we didn't need to explain it. That assumption was wrong. Some expected an EPC before the building had been constructed, which isn't how it works — the certificate follows the completed dwelling, not the drawings.
What works better is setting out, at the outset, what information is needed and when:
Drawings
An outline fabric specification
The intended heating system
That's enough to sketch the CO2 position early. If it clears the target with a comfortable margin, the rest of the detail can follow at a more relaxed pace. If it's failing, or only narrowly passing, that's the point to start working through design or specification alternatives — before construction, not after.
5. Thermal bridging can be the difference between a pass and fail
This one is easy to underestimate. If a project is on a tight budget, using a recognised, accredited construction detail avoids the cost of a full bespoke assessment — and avoids having to claw back the loss elsewhere in the build. A poor detail, assessed using default values, can carry a heat loss penalty large enough to undo fabric improvements made everywhere else on the building. It's a small line item in the specification with an outsized effect on the result.
A poorly detailed junction can undo fabric improvements made everywhere else on the building.
6. SAP compliance and a genuine decarbonisation case are two different things
Increasingly, councils want more than a passing SAP result — they want to see that alternatives were properly considered. An energy statement is the document that does that: a record of the technologies explored, what was ruled out and why, and what was ultimately chosen for the site. It doesn't need to be exhaustive. It needs to show the reasoning was real.
There's a useful side effect to writing this properly the first time: it's often the moment something gets noticed that would otherwise have been skipped. It's also worth keeping on file beyond the planning submission — it's a useful reference for the consulting team through the build, and for whoever occupies the building afterwards.
Interestingly, on one conversion project we worked on — commercial premises converted to residential — neither the council nor Building Control asked for a SAP calculation at any stage. The client completed the entire project, got Building Control sign-off, and only then came to us for an EPC. It was surprising at first, but we've since seen it often enough to know it's a common gap, not a one-off. Passing through the system without ever running a SAP calculation doesn't mean the design was optimised for carbon — it just means no one asked the question. This is exactly why getting compliance and genuinely reaching net zero are two different things.
7. None of this holds up without ongoing communication
Every one of the points above traces back to the same root cause: information not reaching the right person at the right time. A locked-down specification only stays locked down if everyone making decisions on site — including advisors brought in after the SAP model is built — knows that a change has to be checked before it's implemented, not reported after the fact. That means communication can't stop once the initial energy strategy is signed off; it needs to continue at the same level through to completion.
The bigger shift: SAP itself is changing
Worth having in view while working through any of the above: the government confirmed the Future Homes Standard on 24 March 2026, with the regulations coming into force early next year, followed by a transition period running for a year. The new standard targets around 75% lower carbon emissions than 2013-standard homes, largely through low-carbon heating and on-site renewables.
As part of that change, SAP is being replaced by the Home Energy Model (HEM) — but not overnight. The government's own consultation response is explicit that both HEM and a version of SAP will remain available for a limited period, to allow a smooth transition rather than a hard cutover. In practice, that means the two methodologies will run in parallel for a time, and which one applies to a given project will depend on when the application was submitted and where it sits relative to the March 2027 and March 2028 dates.
For any scheme with a live or upcoming planning condition, that means two things worth checking now: which methodology and rule set will actually apply by the time the application is determined and construction starts, and whether the heating and renewables strategy being locked in today still makes sense under the standard that's coming.
Need a SAP calculation or energy statement for a live planning condition?
PassiHVAC works across the full range of residential development — from single self-builds through to larger multi-unit schemes — taking projects from initial CO2 emission modelling through to Part L compliance, energy statements and GLA emissions reporting. We also work alongside consultancies as additional capacity where an extra pair of hands is useful, rather than only as a standalone service. Get in touch to discuss your project.
Frequently asked questions
What is a SAP calculation and when do I need one?
A SAP (Standard Assessment Procedure) calculation is the method used to demonstrate that a new dwelling in England and Wales meets Part L of the Building Regulations. It's typically required at both the design stage (to inform planning and Building Control) and as-built (to produce the EPC on completion).
Why has my council asked for an energy statement as well as a SAP calculation?
SAP demonstrates regulatory compliance. An energy statement is a separate document, often required at planning stage, that sets out the reasoning behind the energy and carbon strategy — what technologies were considered, and why the final approach was chosen. Councils increasingly ask for both.
Is SAP being replaced?
Yes, but gradually. As part of the Future Homes Standard, confirmed by the government on 24 March 2026 and coming into force from 24 March 2027, SAP is being replaced by the Home Energy Model (HEM) as the method for demonstrating compliance. The government has confirmed that both SAP and HEM will be available in parallel for a limited transition period, so this is a phased changeover rather than an overnight switch — which project a given scheme falls under will depend on submission and construction timing relative to the March 2027 and March 2028 dates.
How much can thermal bridging affect a SAP result?
Significantly. Using default (non-accredited) values for junction details can carry a heat loss penalty large enough to offset fabric improvements made elsewhere in the building. Using a recognised, accredited construction detail avoids this and reduces assessment cost.
When should SAP calculations start in a project?
As early as possible — ideally alongside the initial design, before layouts and specifications are fixed. Starting early leaves room for low-cost changes (such as facade or roof pitch adjustments) that can materially improve the result.
What is PassiHVAC's approach to SAP calculations and energy statements?
We keep it simple. First, we set out a plan of action based on what stage the development has reached and what the local council is asking for. We then collect the basics — planned fabric, construction, drawings, heating system, hot water, and any renewable opportunities on site. Once we've assessed these, we present a clear report showing where the design currently stands.
From there, there are two paths. If the design already meets the criteria, we move on to the finer detail — lighting, ventilation, energy-saving features, thermal bridging, and so on. If it doesn't, we set out a few options to adjust the as-designed choices until it passes.
What's the difference between a SAP calculation and an EPC?
A SAP calculation is the underlying assessment; the EPC is one of its outputs. Building Regulations require a SAP calculation and a predicted energy assessment before work starts, and a further SAP calculation once the dwelling is complete, reflecting the as-built specification and air pressure test result. That final calculation is what produces the EPC — the certificate that, by law, must accompany a new home when it's marketed for sale or rent.
Do I need a SAP calculation for an extension or conversion, or only for new builds?
New dwellings always need one. Conversions — turning a commercial building into residential use, or splitting a single dwelling into flats — usually need one too. Extensions can trigger the requirement as well, particularly where glazing makes up more than around a quarter of the extension's area, since that level of glazing changes the heat loss picture enough to need checking against Part L. For many cases a full SAP may not be needed, and a short report explaining the planned upgrade, thermal improvements, U-value calculation or comparison to a notional building may just work.
What's the difference between an as-designed and an as-built SAP calculation?
The as-designed calculation is based on the drawings and specification at the point of application — it's what gets used to satisfy a planning condition or Building Control at the outset. The as-built calculation is revisited once construction is complete, updated for the actual air pressure test result and any specification changes made on site, and it's this version that generates the final EPC. The two can diverge significantly if changes go in without being checked against the model first — which is exactly the risk described above. One may end up paying a higher carbon offset payment to the council.
Is SAP the right tool for my project, or do I need SBEM?
SAP applies to residential dwellings. Non-residential buildings, and mixed-use schemes with a non-residential element, are assessed using SBEM (the Simplified Building Energy Model) or, for more complex geometries, dynamic simulation modelling (DSM).
Comments