PAS 2035 Explained (2026): What UK Homeowners and Architects Actually Need to Know

Why PAS 2035 exists

To understand PAS 2035 you have to understand what went wrong before it. Between roughly 2009 and 2018, the UK ran a series of large-scale energy efficiency programmes — CERT, ECO1, ECO2, ECO3, the Green Deal — that funded millions of insulation installations in privately owned and rented homes. A meaningful proportion of those installations went badly wrong. The most common failures were cavity wall insulation in unsuitable properties, external wall insulation detailed without thinking about junctions, and any insulation work done without considering ventilation.

In 2015 the government commissioned the Each Home Counts review (Bonfield, December 2016), led by Peter Bonfield, which reported in December 2016 and concluded that the UK retrofit industry needed a quality framework that treated the whole house as a system. PAS 2035 — Publicly Available Specification 2035 — was the BSI document that resulted. The first version was published in 2019; the current major revision is PAS 2035:2023.

For APMBuild's UK clients in 2026 — homeowners, architects, developers — PAS 2035 is the single most important document in the UK retrofit landscape, even when it does not legally apply to their project.

What PAS 2035 is — and what it isn't

PAS 2035 is a process framework. It defines how a retrofit project should be assessed, designed, coordinated, installed and evaluated. It does not specify performance targets the way EnerPHit does. It does not require you to hit a particular U-value. What it requires is that whoever is making decisions on your project is competent, that the decisions are documented, and that the project considers the whole building rather than individual measures in isolation.

The defining principle is "whole-house retrofit". The standard recognises that you cannot insulate walls without thinking about ventilation, that you cannot replace windows without thinking about moisture, and that you cannot install a heat pump without understanding heat loss. Every measure interacts with every other measure.

PAS 2035 is also not a quality stamp on the finished product. It is a quality discipline applied to the process. The performance of the result still depends on the targets that were set, the design that was chosen and the workmanship that was delivered.

PAS 2030 vs PAS 2035

These two documents are commonly confused. They are different things and they work together.

The practical relationship: a PAS 2030 installer works within a PAS 2035 project. The project framework (2035) sets the design intent and the boundary conditions; the installer standard (2030) ensures the installation matches what was specified.

When PAS 2035 applies

Mandatory:

• Any retrofit work funded by ECO4 (the Energy Company Obligation).
• The Great British Insulation Scheme (GBIS).
• Help to Heat / Home Upgrade Grant funded works.
• Most social housing retrofit programmes (typically required by housing associations).
• Retrofits where the contractor or installer wants to maintain their TrustMark accreditation.

Not mandatory but recommended:

• Privately funded EnerPHit retrofits.
• Deep retrofits on listed buildings (where moisture risk is highest).
• Any retrofit involving multiple coordinated fabric measures.

On APMBuild's privately funded deep retrofit projects we typically follow PAS 2035 documentation discipline by default. It costs a fraction of the project total and it prevents the most common failures. For ECO4 projects, our supply chain partners include certified PAS 2030 installers and the documentation is mandatory.

The five specialist roles

PAS 2035 names five roles. On a small project one person can cover several; on a larger or higher-risk project they are usually different people.

1. Retrofit Assessor

Conducts the initial whole-house survey. Records building condition, occupancy, ventilation, heating, moisture risk, listed building status. The output is the Retrofit Assessment report.

2. Retrofit Designer

Takes the Assessment and produces a Retrofit Design — fabric build-ups, junction details, ventilation strategy, moisture risk analysis. On EnerPHit projects this person typically works closely with the PHPP modeller and the architect.

3. Retrofit Coordinator

The central role. Holds responsibility for the integrity of the whole project. Approves the design, signs off the install, and is accountable to TrustMark. The Coordinator is the single point of professional liability.

4. Retrofit Installer

The contractor who actually fits the measures. Must hold PAS 2030 accreditation for each measure type they install.

5. Retrofit Evaluator

Conducts the post-installation check — visual, performance and (on higher-risk projects) sensor-based moisture monitoring. Reports back to the Coordinator and TrustMark.

APMBuild works within this framework on PAS 2035 projects, partnering with accredited Retrofit Coordinators and Designers. See our full service list for how this integrates with our standard delivery, and our materials catalogue for the components routinely used on PAS 2035-compliant builds.

The PAS 2035 documentation flow

A compliant PAS 2035 project produces six core documents. On ECO-funded works these are uploaded to TrustMark's data warehouse and become the audit trail.

1. Retrofit Assessment. Whole-house condition survey, occupant interview, ventilation check, moisture risk screening, listed building status, heritage considerations.
2. Improvement Option Evaluation. Cost-benefit analysis of possible measures, including unintended consequences.
3. Medium-Term Improvement Plan (MTIP). The sequenced plan of measures over time — critical because most retrofits happen in phases.
4. Retrofit Design. Detailed design package — fabric build-ups, junction details, ventilation strategy, U-values, moisture analysis.
5. Installation records (PAS 2030). Photographs, test results, materials data, installer competence records.
6. Retrofit Evaluation. Post-completion check. On higher-risk projects this includes airtightness testing and moisture monitoring.

For homeowners, the practical implication is that you should expect to see all six of these documents on any PAS 2035 project. If your contractor cannot produce them, the project is not PAS 2035-compliant regardless of what the paperwork says.

PAS 2035 vs EnerPHit

These two systems address different parts of the retrofit problem. PAS 2035 is about how to run the project; EnerPHit is about what performance the building must reach. They are complementary.

APMBuild's view: on any deep retrofit, both make sense. EnerPHit sets the performance ambition; PAS 2035 keeps the process honest. The combination is what produces buildings that work as designed.

Common PAS 2035 mistakes

Even with PAS 2035 in place, the same recurring mistakes appear on UK retrofit sites.

Skipping the ventilation assessment

PAS 2035 requires ventilation to be assessed before any insulation measure. Many ECO4 projects still treat ventilation as an optional add-on. The result is condensation and mould in homes that were dry before retrofit. Read the UK government guidance on condensation and mould for context.

Ignoring moisture risk in solid walls

Solid walls (pre-1919 housing) handle moisture differently from cavity walls. Internal wall insulation done without hygrothermal analysis can drive condensation behind the insulation board. PAS 2035 requires this analysis; in practice it is sometimes skipped.

Wrong U-values from out-of-date tables

U-values in older publications often use conservative defaults that lead to over-spec'd insulation or, more dangerously, miss the real heat-loss hotspots. APMBuild calculates U-values per element, using PHPP-validated build-ups and current manufacturer datasheets.

Pairing PAS 2035 with Passive House principles for premium retrofit

The strongest UK retrofit projects we have seen — and the projects APMBuild aims to deliver as standard — combine PAS 2035 discipline with Passive House physics. The PAS 2035 documentation flow gives you process integrity. The Passive House (or EnerPHit) targets give you the performance ambition. PHPP modelling — see our PHPP guide — gives you the design accuracy to verify that the targets are achievable.

Practically, this means a typical APMBuild deep retrofit will produce:

• A PAS 2035 Retrofit Assessment and Design package (process integrity).
• A PHPP pre-assessment confirming EnerPHit feasibility (performance verification).
• A specified fabric build-up using Polish, German or Austrian materials with full datasheets (supply chain integrity, typically 5–10 day delivery).
• A blower door test at completion (workmanship verification).
• APMBuild's 24-month workmanship warranty (commercial accountability).

Each layer addresses a different failure mode. Together they make the result reliable.

Future direction — Part L uplift and beyond

The regulatory direction of travel matters because retrofit decisions made in 2026 will sit alongside increasingly demanding building standards in 2028–2030.

The next Part L uplift — bundled into the Future Homes Standard package due in force on 24 March 2027 — pushes fabric and renewables further than the current Part L 2021 (with 2023 amendments). New-build housing will be expected to operate at carbon levels close to net zero, with heat pumps and improved fabric as the default.

The Future Homes Standard, now scheduled to come into force on 24 March 2027 (per Building Circular 01/2026), sets the trajectory for new homes — no gas boilers in new homes, much improved fabric, and a step toward operational net zero.

Minimum EPC C for private rentals — currently consulted on with a 2030 compliance deadline for all tenancies, and new tenancies required to meet the standard from 2028 — would require landlords to bring properties up to EPC C, which for most pre-1980 stock requires deep retrofit, not cosmetic upgrades.

MMC (Modern Methods of Construction) is increasingly used on retrofit and new-build projects — prefab insulation panels, factory-finished window cassettes, off-site timber frames. APMBuild incorporates MMC components from European manufacturers where they improve quality and speed.

The combined effect is that retrofit decisions made today should be future-proof against regulations that do not yet exist. PAS 2035 + EnerPHit + PHPP is currently the most reliable combination for that.