SAP or PHPP: which figures belong in an energy statement

A planning energy statement needs the figure the policy asks for, and in England that is almost always a SAP output. The Passive House Planning Package tells you far more about how a building will behave, but no English development plan policy is written in its units, so a Passivhaus scheme still has to produce a SAP based carbon or primary energy comparison for the application. Submit PHPP alone and you have given the case officer evidence they cannot score against the policy.

That is the practical answer. The interesting part is why the two sets of numbers cannot be converted into each other, and what to do when a well meaning officer writes a condition mixing them up.

What each method is built to do

SAP, the Standard Assessment Procedure, is a compliance tool. It exists to compare a proposed dwelling against a notional dwelling of the same size and shape built to a defined standard, and to produce the outputs the Building Regulations and the energy certificate regime need. Occupancy, internal temperatures, hot water demand and ventilation rates are standardised, because the point is comparability between dwellings rather than prediction of a particular household's bills.

PHPP is a design tool. It is an annual and monthly energy balance using local climate data, with the designer's own shading, thermal bridge and ventilation inputs, aimed at hitting the Passivhaus criteria: a space heating demand at or below 15 kWh per square metre of treated floor area per year, a heating load at or below 10 watts per square metre, airtightness at or below 0.6 air changes per hour at 50 pascals, and a limit on the frequency of hours above 25 degrees.

Different purposes, different conventions, and the conventions are where the trouble starts.

Why the numbers do not convert

Three mismatches make a straight comparison meaningless.

The floor area is not the same. PHPP uses treated floor area, which excludes stairs, walls and unheated space and applies reductions to circulation. SAP uses total floor area measured internally. A space heating demand expressed per square metre of treated floor area is therefore a larger number than the same building's demand per square metre of SAP floor area, for reasons that have nothing to do with the building's performance.

The scope is not the same. Part L and SAP deal in regulated energy: heating, hot water, ventilation, pumps, fans and fixed lighting. The Passivhaus primary energy renewable figure includes household electricity and appliances. A scheme can look better under one and worse under the other purely because of what is counted.

The method is not the same. SAP applies standard occupancy and fixed assumptions by design. PHPP uses the designer's inputs and local climate, so two competent modellers will produce closer SAP results than PHPP results for the same building, and PHPP results closer to what the building actually does.

The figure your local policy asks for

Read the policy, not the explanatory text. English development plan energy policies are usually written in one of three ways: a percentage improvement on the Part L target emission rate or primary energy rate, an absolute carbon or energy intensity figure, or a requirement to follow a hierarchy and quantify each stage.

In London, Policy SI 2 of the London Plan 2021 sets a minimum on site carbon reduction beyond the Building Regulations baseline, with the remainder offset through a cash in lieu contribution, and the Greater London Authority's energy assessment guidance prescribes the format of the figures. Both the percentage and the offset calculation come out of a SAP or, for non-domestic floorspace, an SBEM model. PHPP has no route into that arithmetic.

Outside London, check whether the policy still refers to a superseded edition of Part L. Many adopted plans predate the 2021 uplift and express their requirement against the 2013 baseline, which makes the percentage meaningless until someone restates it. Say so in the statement and show both baselines rather than silently choosing the easier one.

Presenting both without confusing the officer

The format that works is a two part Energy Statement. Part one answers the policy in the policy's own units, with the SAP outputs, the baseline edition named, and a table showing each stage of the energy hierarchy with its percentage saving. Part two presents the PHPP evidence as design assurance: fabric values, thermal bridge treatment, airtightness target, space heating demand and overheating frequency.

Two rules keep it readable. Never put a PHPP number in the policy compliance table, and never let a SAP number appear in the Passivhaus section without saying which it is. Label every figure with its method and its floor area basis on the same line. An officer who has to work out which model a number came from will ask for a revision, and that is three weeks.

A short paragraph explaining that the two sets of figures are not comparable, and why, is worth including. It pre-empts the objection that the heating demand in one table does not match the energy figure in the other.

Overheating is where the two methods argue

Approved Document O governs overheating in new residential buildings for Building Regulations purposes, with its simplified method and its dynamic thermal modelling route. The Passivhaus criterion is a frequency of overheating expressed as a percentage of hours above 25 degrees in PHPP. CIBSE TM59 is a third method again, and the one many London boroughs ask for.

A building can pass one and fail another, which is why an Overheating Risk Assessment should state which method it used and against which criterion. Councils expect a suitably qualified author for this work and professional review is strongly advised, particularly where the facade is noisy and the openable area needed for purge ventilation conflicts with the acoustic specification.

On a Passivhaus scheme the honest position is usually that PHPP is the design driver and the Approved Document O or TM59 output is the compliance evidence. Say that plainly rather than presenting one as if it satisfied the other.

Certification timing against the planning programme

Passivhaus certification is awarded after construction, by a certifier, on the basis of as built PHPP inputs, airtightness tests and commissioning records. And nothing about it can be delivered at planning stage. What you can offer at application is a PHPP model of the design, a statement from a certified Passivhaus designer, and a commitment to certification.

If a council wants that commitment secured, the mechanism is a condition or an obligation requiring evidence of certification within a period after practical completion, with a fallback if certification is not achieved. Negotiate the fallback. A condition requiring certification with no alternative route creates a breach that nobody can remedy if a single thermal bridge misses, and the authority gains nothing from it.

The parallel point for a commercial scheme, where the energy evidence base works differently again, is covered in the piece on energy statements for warehouse and logistics schemes.

Conditions that name the wrong metric

The condition to watch for reads something like: "the development shall achieve a space heating demand not exceeding 15 kWh/m²/year as demonstrated by SAP calculation". SAP does not produce that figure on that floor area basis, so the condition cannot be discharged as drafted.

Variants of the same problem appear regularly:

• A carbon reduction percentage with no baseline edition named, so nobody knows whether it is against the 2013 or 2021 Part L target.
• An airtightness figure quoted in cubic metres per hour per square metre at 50 pascals in one clause and air changes per hour at 50 pascals in another, which are different quantities.
• A requirement for an energy certificate band plus a primary energy figure, where the two are inconsistent for the specification proposed.
• A requirement to achieve certification "prior to commencement", which is impossible.

Deal with these at the draft condition stage. Ask the case officer for the draft conditions before the decision issues, which most authorities will share on request, and offer replacement wording that names the metric, the method, the floor area basis and the evidence. Officers accept that correction almost every time, because the alternative is a discharge application they cannot determine. Once the decision notice is issued your options narrow to a section 73 application to vary the condition, and the Sustainability Statement that accompanies it is work you should not have had to pay for twice.

Part L, Approved Document O and the Future Homes Standard are all on their own revision cycles, so check the current editions and any transitional arrangements on gov.uk before relying on a figure in a submission.

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