PHPP Modelling for UK Architects: When to Use It, What It Costs, and How to Read the Output

Why PHPP matters for UK low-energy briefs

The UK architecture market has shifted decisively over the past five years. Clients arriving with a brief that explicitly references net zero operational carbon, EPC A targets, EnerPHit certification or LETI-aligned performance are no longer fringe. They are the new mainstream of mid-to-upper-end residential work.

The problem is that SAP — the modelling tool every UK architect knows — was never designed to support low-energy design. SAP exists to demonstrate Building Regulations compliance and to generate EPCs. It is reasonably accurate when fabric performance is average. It becomes systematically misleading when fabric performance is high.

PHPP fills that gap. It is the official tool for Passive House Institute (PHI) certification and the most rigorous energy model available for low-energy buildings in Europe. APMBuild Ltd, based in Hereford and serving architects across the West Midlands and Wales, uses PHPP as the design tool of choice on projects targeting heating demand below 30 kWh/m²a — whether or not the client pursues formal certification.

The UK Passive House market is still small but growing fast. Around 1% of new UK homes were built to Passivhaus standard in 2025, with the Passivhaus Trust targeting 10% by 2035 (Passivhaus Trust, February 2025). Wales is one of the UK's strongest Passivhaus regions — 23 Welsh social housing providers have adopted it as their enhanced standard via the Tai ar y Cyd Pattern Book — and Welsh borders sit firmly within APMBuild's service area.

PHPP in the 2026 UK regulatory landscape

Three regulatory shifts in 2026 made PHPP more relevant to UK architects, not less.

MEES 2030 — the dual-metric standard

The Warm Homes Plan (21 January 2026) confirmed a new compliance framework for the private rental sector from 1 October 2030: a primary Fabric Performance metric plus a choice of Heating System or Smart Readiness metric. PHPP is the only widely-used tool that models all three in a single integrated balance — fabric loss, heating system performance and PV/battery contribution. SAP treats them as separate calculations and frequently disagrees with itself across the boundaries. See our 2026 MEES Reset article for the implications for landlords and PRS designers.

SAP → HEM cutover (1 October 2029)

From 1 October 2029, new EPC assessments in England and Wales move from SAP to the Home Energy Model (HEM). HEM is closer in philosophy to PHPP — a steady-state model with better thermal bridge handling and more realistic ventilation modelling. The official UK methodology is moving toward PHPP, not away from it. For projects designed today that complete after October 2029, PHPP is the closest thing to a forward-compatible energy model.

Future Homes Standard, 24 March 2027

The Future Homes Standard (FHS) pushes Building Regulations Part L further toward fabric-first design, with new homes expected to produce roughly 75% less carbon than 2013 standards. The fabric U-values implied by FHS sit close to PHPP's design space — at that level SAP's defaults become misleading and PHPP becomes the natural design tool. We covered the wider Part L trajectory in our PAS 2035 article.

Combined, these three shifts mean a UK architect briefing a project for completion after 2027 is now substantially more likely to need PHPP than the same project briefed in 2024 would have been.

What PHPP actually is

PHPP is an Excel-based calculation tool published and maintained by the Passive House Institute in Darmstadt, first released in 1998. The current release is PHPP 10 (10.x major version), updated roughly every 2–4 years. It is the official tool for PHI Passive House and EnerPHit certification — and it is used widely by European architects who never seek formal certification, simply because it is the most rigorous energy model available for low-energy buildings.

What makes PHPP different from SAP is not what it calculates but how. PHPP is a steady-state monthly balance model with substantial empirical calibration. It accounts for:

• Thermal bridges (ψ-values) at every junction.
• Real ventilation system performance, including duct losses and efficiency drift.
• Climate-specific solar gains, drawn from European meteorological datasets.
• Window-by-window orientation, frame factor, glazing g-value and shading.
• Internal gains based on residential occupancy patterns refined from PHI post-occupancy monitoring since 1996.
• Summer overheating frequency under realistic warm-weather scenarios.

The output is a set of monthly heating and cooling demand figures, an annual heat demand in kWh/m²a, a primary energy demand, an overheating frequency, and U-values for every element of the envelope. If you understand the report, you can iterate the design with surgical accuracy.

PHPP vs SAP — the comparison

The practical implication: on a Passive House design, SAP frequently over-predicts heating demand by 20–50% compared with PHPP. The architect specifies more heating capacity than the building actually needs, the client overpays for kit, and the building can end up running hot in summer. This pattern is documented in post-occupancy studies by the Passivhaus Trust UK and the AECB — the gap is one of the strongest practical reasons for using PHPP at design stage on any genuinely low-energy brief.

When you need PHPP on a project

Not every UK residential project needs a PHPP model. Here is APMBuild's practical decision tree.

You need PHPP if any of the following are true:

• The brief specifies Passive House or EnerPHit certification.
• The target heating demand is below 30 kWh/m²a.
• The client wants verified net zero operational carbon.
• The project is highly glazed and there is any overheating concern.
• You are designing for a JV or development partner who will scrutinise the energy model during due diligence.
• You want to specify MVHR confidently rather than as a black box.

SAP alone is sufficient if:

• The project targets Building Regulations compliance only.
• EPC band C or D is acceptable to the client.
• The fabric performance is conventional (U-values ≥ 0.25 W/m²K).
• The project is owner-occupied and will not be let after October 2030 (see MEES caveat below).

MEES caveat for landlords and PRS work: from 1 October 2030, the MEES dual-metric standard applies to private rentals. The new Fabric Performance metric is closer in spirit to a PHPP-style fabric assessment than to a SAP rating. Even a "conventional" PRS retrofit benefits from PHPP-style fabric thinking at brief stage — see our 2026 MEES Reset article.

If you are unsure, APMBuild offers a free 30-minute call to look at the brief together. See our PHPP service page for the standard pre-assessment package.

Inputs PHPP requires

PHPP needs more information than SAP. The trade-off is that it gives more reliable output. Here is what a competent PHPP assessor will ask the architect to provide:

Geometry

Treated floor area (the official Passive House definition is slightly different from GIA — known as TFA). External envelope areas broken down by orientation. Net glazing areas for every window. Internal volumes for ventilation calculations.

Fabric

U-values for every element of the envelope, including the build-up. Where the build-up is not yet finalised, the assessor can iterate. Thermal bridge ψ-values at every junction — the architect's details package needs to be specific enough to read these.

Glazing

Whole-window U-value (Uw), glass U-value (Ug), frame U-value (Uf), spacer ψ-value (ψ-spacer), glass g-value, frame factor. Reputable European window manufacturers — particularly those in Poland, Austria and Germany — provide all of this on a single product datasheet. APMBuild's direct European supply chain gives the design team PHPP-compatible glazing data as standard.

MVHR specification

Heat recovery efficiency (per PHI testing, not manufacturer marketing). Specific fan power. Duct layout and length. Location of the MVHR unit relative to the thermal envelope.

Location and climate

Site postcode, altitude, ground conditions, orientation. PHPP includes UK climate datasets for all major regions.

How to read a PHPP report

A complete PHPP output runs to dozens of pages, but architects should focus on five core numbers. Once you can read these, you can interrogate the design.

1. Annual heating demand (kWh/m²a)

The headline number. Targets: Passive House <15, EnerPHit <25–30 depending on method, ambitious UK net zero <30–40. Anything above 50 means the design is not low-energy regardless of marketing.

2. Annual primary energy demand (kWh/m²a)

Accounts for all energy use including domestic hot water, lighting and appliances, weighted by primary energy factors. Passive House target: <120 kWh/m²a (or PER <60 for the new methodology — see Passipedia: Classic, Plus, Premium Passive House classes).

3. Heating load (W/m²)

Peak heating load on the coldest day. Passive House target: <10 W/m². This number drives heating system sizing. If you see 25 W/m² in the report and the architect has specified a 5 kW heat pump for a 200 m² house, the numbers do not match.

4. Overheating frequency (% hours over 25°C)

Passive House limit: <10% of hours. In UK practice with increasing summer temperatures, target <5%. If the report shows 15%+, the glazing strategy needs work — usually external shading or reduced south glazing.

5. Airtightness (n50, ACH)

Design target. PH: 0.6 ACH@50Pa. EnerPHit: 1.0 ACH@50Pa. UK Building Regs (Part L 2021): 8 m³/(h·m²) @ 50 Pa air-permeability backstop. The gap between the latter two tells you how much detail and discipline the build requires.

Every element table at the back of the report shows the U-value, the area, and the heat loss in W/K. This is where you find the weakest element to improve first.

Common pitfalls in UK projects

From projects APMBuild has reviewed for UK architects, three pitfalls recur.

Form factor under-estimation

Form factor — external envelope area divided by treated floor area — dictates how easy or hard the energy targets are. A form factor below 2.5 is comfortable; above 3.5 is hard. UK detached bungalows often hit 4+. PHPP will reveal this immediately; SAP-only models hide it.

Thermal bridges treated as defaults

UK practice often uses Approved Document L default ψ-values. PHPP wants real calculated values. The difference between "default 0.08" and "calculated 0.04" across 50 metres of wall-to-roof junction is 2 W/K — real money in energy and real risk of cold-spot condensation.

Glazing g-value chosen for daylight, not heat

UK architects often specify low-g glazing (0.4) for solar control. In a properly shaded south facade, a high-g glazing (0.6) gives free winter heat and PHPP will reward it. The right choice is project-specific and only PHPP shows the trade-off.

Working with a contractor who can read PHPP

A PHPP report is an instruction manual for the build. If the contractor cannot read the manual, the as-built fabric will not match the model. The two most common breakdowns:

• Insulation continuity at junctions — the contractor places a service void where the model expected continuous insulation.
• Airtightness layer punctured by trades unaware of its location — the model expects 0.6 ACH; the test result is 2.5 ACH.

APMBuild's director, Paweł Okurowski, is a Passive House-trained builder (PHI-accredited Level 2 training, AZB Wrocław, January 2023) with 20+ years of European construction experience across Poland, Germany, Spain and the UK. He carries out PHPP pre-assessments in-house using PHPP methodology, reads full PHPP reports as standard design input, builds to them, and verifies the result with a blower door test before handover. For UK architects, this means the design model you commission actually arrives in the finished building. Read more on our architect partnership page, or browse our materials catalogue to see the European components we routinely specify with PHPP-compatible datasheets.

Typical costs and timelines

UK PHPP pricing varies by complexity and certification route. APMBuild splits the work between in-house pre-assessment by Paweł and full certified modelling delivered via our PHI-accredited partner:

• PHPP pre-assessment — from £750 + VAT. Carried out in-house by Paweł, using PHPP methodology (PHI-accredited Level 2 training, AZB Wrocław 2023). Indicative model used during early design to confirm the brief is achievable and to set fabric and MVHR targets. 1–2 week turnaround. Useful at RIBA Stage 1–2 to inform the client's budget conversation and the architect's concept work.
• Full PHPP model for design — £1,800–£2,800 + VAT, depending on geometry complexity. Delivered via our PHI-accredited modelling partner, with Paweł coordinating between architect, client, partner and site. 2–3 week turnaround. Used to finalise fabric, glazing and MVHR specification at RIBA Stage 3–4.
• PHPP for Passive House or EnerPHit certification — £2,500–£3,800 + VAT. Delivered via our PHI-accredited modelling partner with full QA against PHI requirements. Allow 3–4 weeks plus PHI certifier review.

These figures are 2026 indicative rates and look high next to a simple SAP at £200–£400. On a £400k+ build the PHPP cost is a fraction of one window's cost and removes design-stage risk worth multiples of itself.

APMBuild PHPP pre-assessments are indicative and intended to guide specification decisions — they are not produced by a PHI Certified PHPP Expert or Certified Passive House Designer. Where full certified PHPP modelling is required for building certification, APMBuild works with an accredited PHI partner.