PAS 2035 Retrofit Delivery: Technical Preparation for Warm Homes Wave 3

With funding awarded and delivery now underway for Warm Homes Wave 3, getting the technical detail right matters more than ever. In two recent contributions to the RISE (Retrofit Information, Support and Expertise) programme – funded by DESNZ – Jack Allen, Managing Director and Founder of Building Energy Experts, sets out the assessment, coordination and design practices needed to successfully deliver PAS 2035 compliant Warm Homes projects at scale.

For local authorities, housing associations and delivery partners moving from funding into delivery, the difference between a smooth rollout and a stalled one usually comes down to preparation: the right roles in place early, survey data that can be trusted, and a coordination plan that holds up at scale. Drawing on over 14 years in sustainability within the built environment, Jack outlines the common delivery risks and how to mitigate them through accurate data, effective sequencing and early stakeholder engagement.

In this blog, you’ll learn:

• Critical roles and responsibilities for delivery, including contractor and installer selection
• Why survey quality and BS 40104 compliance are non-negotiable
• How archetype-based assessments streamline programmes, with a real phased-delivery example
• What robust testing and diagnostics actually require, discipline by discipline – including a moisture testing requirement that’s easy to miss
• Why long-term stock strategy beats scrambling to fit properties into a funding wave
• Funding rules and coordination tips to avoid delays

Getting Roles and Responsibilities Right, Early

Getting the right people in the right places, from the outset, is one of the clearest predictors of a scheme’s success. That starts with advisors and consultants who understand the whole process – from initial data through to installation and post-occupancy monitoring – and it’s worth noting these don’t have to be the same people who go on to deliver the project; their job at this stage is simply to make sure the strategy is right.

Updates in PAS 2035:2023 have shifted the responsibilities of the Retrofit Coordinator, ho is effectively the linchpin joining every other element of a project together – making sure assessors are gathering the right information, designers receive a coordination package they can actually work from, and every delivery partner understands what evidence they’re expected to bring. A coordinator’s background matters less than their specific retrofit knowledge: being a qualified architect doesn’t automatically mean someone understands the full range of applicable domestic retrofit measures, and people from an architectural technologist or technical specification background are often better suited to the role.

Retrofit designers need the same breadth – technical awareness of the different measures, their risks and their benefits across different dwelling types, not just familiarity with one or two.

The final role worth getting right is the installer or contractor delivering the work on site. Quality here varies enormously: some contractors coordinate several trades to complete a full job within a single week, causing minimal disruption and producing TrustMark-compliant evidence as they go; others show up in a disorganised, drawn-out sequence over a month or more, causing more disruption to residents and creating genuine compliance risk through poor evidence. Choosing installers on the same rigour as any other role in the process – not just on price – is worth the effort.

Survey Quality and BS 40104 Compliance

Successful projects hinge on data and survey quality at the initial stages. This wave, assessments also need to meet BS 40104 – the standard covering pre-retrofit assessment methodology for domestic dwellings – which requires considerably more data to be collected than before. That means BS 40104-compliant Retrofit Assessments from the start, not a compliance step bolted on later – poor-quality assessments lead directly to delays, inaccurate designs, and ultimately fewer measures actually getting installed.

Poor data isn’t hypothetical. Previous waves have seen funding applied to properties that turned out to be leasehold and already sold, static caravans that were never retrofittable in the first place, and EPCs up to ten years out of date – sometimes because a window or loft scheme had already been carried out since the EPC was lodged, quietly moving the property’s real rating without anyone updating the record. Any of these means money spent on a survey that can’t actually be used. Worth flagging too: a standard software being accepted for lodgement with a compliance body doesn’t in itself guarantee compliance, so lodgement can’t be treated as a quality check. As Jack put it:

Successful projects hinge upon data and survey quality at initial stages. If you understand your whole stock and how it performs, you can maximise the benefit when funding schemes arise.

— Jack Allen, Managing Director & Founder, BUilding Energy Experts

Archetype-Based Assessments for Scalable Delivery

Archetyping lets programmes stage assessments strategically rather than surveying every property individually from a standing start – good desk-based archetyping can be done digitally, though it still needs validating on site, since no two properties behave identically even when they look similar on paper.

PAS 2035:2023 goes further and actively encourages an archetype-first approach: complete the full retrofit assessment, coordination and design for a representative archetype property first, then apply that notionally across the matching properties in the stock, checking each one on site to confirm it genuinely matches before proceeding. Done well, this unlocks a genuinely phased delivery model rather than an all-or-nothing one: the first batch of properties (in one worked example, around 20) can be fully assessed, coordinated, designed, on site and delivered within roughly 12–15 weeks, after which further batches can move through the pipeline weekly, matched to whatever pace the delivery partner can actually handle – rather than trying to survey, coordinate and design hundreds of properties simultaneously before any of them reach site.

Building a Data Standard That Actually Holds Up

Gathering more data only helps if it’s the right data, structured so a coordinator or modeller can actually use it. In practice, that means going well beyond a basic property schedule: a window-by-window record noting each window’s location, dimensions, glazing type, frame type, orientation, trickle vent condition and openability; radiator-level data (size, single or double) so heat pump output can be properly predicted rather than assumed; and floor plans clear enough – ideally to scale – to coordinate a design from without repeated site queries. Where budget allows, 3D models of the building envelope give designers and coordinators something close to having actually walked the site, which noticeably reduces the back-and-forth that follows a weaker survey. This is the thinking behind our own in-house assessment app, built to flex its data requirements project by project and to produce a bespoke report ready for lodgement – built specifically to close the gap between “compliant” data and data a designer can genuinely use.

Gathering more data only helps if it’s the right data, structured so a coordinator or modeller can actually use it. In practice, that means going well beyond a basic property schedule: a window-by-window record noting each window’s location, dimensions, glazing type, frame type, orientation, trickle vent condition and openability; radiator-level data (size, single or double) so heat pump output can be properly predicted rather than assumed; and floor plans clear enough – ideally to scale – to coordinate a design from without repeated site queries. Where budget allows, 3D models of the building envelope give retrofit designers and coordinators something close to having actually walked the site, which noticeably reduces the back-and-forth that follows a weaker survey. This is the thinking behind our own in-house assessment app, built to flex its data requirements project by project and to produce a bespoke report ready for lodgement – built specifically to close the gap between “compliant” data and data a designer can genuinely use.

Testing and Diagnostics That Inform Design

Testing only earns its cost if the results actually feed into the design – treated as evidence-gathering rather than a box-ticking exercise. What “done properly” looks like varies by discipline, and how much of it you need depends heavily on how good your existing data already is: if you already know, for instance, that 300 properties in your stock have unfilled cavities, that tells you to commission a proper borescope inspection on each; if you don’t have that level of detail yet, a good retrofit assessment is the right place to start rather than jumping straight to specialist testing.

Air tightness testing

When it comes to air tightness testing, pulse and blower door testing are often treated as interchangeable, but they’re suited to different jobs. Pulse is quicker and less intrusive, but blower door remains the superior diagnostic tool – depressurising the building lets a trained tester physically trace leakage paths using a smoke machine or thermal imaging, and mark and describe each one rather than handing over a single certificate with no indication of where the problem actually is. This wave also changes the requirement itself: blanket air testing tied to the old risk pathways has gone, and testing is now only required where a measure actually affects air tightness or a specific reduction target has been set – though an air tightness strategy is still required across the board, whether or not a test follows from it. Where a test is needed, commissioning it early and on representative archetypes – rather than leaving it until the end – gives the design team time to actually use the diagnostic detail, not just a pass/fail certificate.

Part F ventilation testing

Despite being a new build requirement for 15 years, ventilation testing remains one of the least understood and enforced elements of Building Regulations. A proper Part F assessment needs a competent person with calibrated equipment (an anemometer or balometer), and goes well beyond simply logging flow rates – trickle vents, door undercuts and purge ventilation all need to be checked too, or the assessment isn’t telling you whether the building is actually compliant.

Moisture and damp testing

An easy requirement to miss: PAS 2035:2023 also references BS 5250 – the code of practice for managing moisture in buildings, covering condensation and air movement – requiring humidity and damp readings wherever a project is planning to change the building’s fabric. Knowing up front that a property has no existing damp or humidity issues lets designers and coordinators progress measures like external wall insulation with confidence; skipping this check risks specifying a fabric change that traps moisture problems the survey never caught.

Thermal imaging

When it comes to thermal imaging, cheap, rushed surveys – low-resolution cameras, wrong time of year, no formal training – routinely lead to the wrong conclusions being drawn from them. A valid survey needs a 10°C internal-to-external temperature differential, which in practice limits the reliable window to roughly October–April; commissioning one in summer is largely wasted effort. Done properly with high-resolution equipment, thermal imaging also supports thermal indexing – comparing internal, external and surface temperatures to identify where a surface will fall below dew point, predicting condensation and mould risk before it becomes visible.

Borescope inspections

Borescope cavity wall inspections are doing as standard on cavity wall properties as part of the retrofit assessment. Confirming whether a cavity is actually filled up front avoids a costly return site visit later, when a design has already assumed an unfilled cavity is available as a straightforward, cost-effective measure.

Getting the Right Assessors in Place

Assessment quality varies more than programmes often expect. The barrier to entry for retrofit assessment work is relatively low, and training course quality varies from genuinely good to poor – it’s possible to move from an unrelated job into assessing buildings within a couple of weeks, working alongside retrofit assessors with two decades of genuine building surveying experience. When awarding assessment work directly, checking qualifications, experience and examples of previous work matters more than defaulting to the lowest quote – the gap in output quality between a strong and a weak assessor is large enough to be worth paying for.

Tenant Engagement and Participation

Resident engagement is just as important as technical design, and it’s one of the areas programmes most commonly get wrong. Letters sent out too late, written without enough information, and with no engagement event to back them up all reliably produce high cancellation rates, low booking rates and properties dropping out of scope entirely. The gap between a well-run and a poorly-run tenant engagement process isn’t marginal – it’s the difference between roughly 40% of residents getting booked in and roughly 90%. Bringing along a resident from a previous, well-received retrofit to speak directly to new tenants tends to build more trust than any letter can.

It’s also worth planning around GDPR contact limits from the outset: there’s a limited number of times you’re able to contact a resident before you need to hand off to a tenant liaison officer, so those early contacts are worth using well rather than rushing to “just get started.” Early, clear communication and realistic timelines directly influence assessment success rates and participation – and, in turn, the whole project’s timeline. As Jack put it:

Engaging with residents early is crucial to ensure a smooth project rollout. Start with clear communication, realistic timelines, and a solid understanding of your building stock.

— Jack Allen, Managing Director & Founder, BUilding Energy Experts

Thinking in Decades, Not Waves

The programmes that get the most value from any given funding wave are the ones that already understand their stock as a whole, with a long-term improvement strategy that spans a decade or two – not just the lifetime of the current wave. Without that, it’s easy to end up forcing the wrong properties into a full PAS process simply because funding has appeared: a property that’s already a high D and only needs loft top-up and LED lighting doesn’t need – and won’t benefit from – thousands of pounds of consultancy fees and a full PAS 2030 installer chain to fit a light bulb. Properties like that are usually better served by a separate, simpler scheme, leaving PAS capacity and funding for the properties that genuinely need it.

Navigating Capital vs. Admin Funding Rules

Understanding what qualifies as capital versus admin (A&A) cost under Warm Homes funding helps programmes maximise grant value and avoid budget surprises later in delivery – retrofit assessment and coordination costs can sit in either category, giving programmes some flexibility in how they invest in survey quality up front. Getting this classification right early is a coordination task as much as a financial one.

What’s Changed Between PAS 2035:2019 and PAS 2035:2023

A handful of structural changes are worth knowing before planning a scheme under the current standard. The old risk pathways have been removed, shifting more oversight and judgement onto the Retrofit Coordinator directly. Monitoring and evaluation requirements have also been strengthened, in line with BS 40101 – the standard covering building performance evaluation in occupied, operational buildings. You don’t need to monitor every property, but monitoring a sample of archetypes before and after works, ideally for at least a year, gives a genuine, quantified read on the energy and wellbeing benefits delivered, rather than an assumption.

The change most likely to get missed is a new requirement to consider climate resilience and adaptation as part of the design – treating cold homes, overheating and damp/mould not as separate problems but as a combined set of risks a whole-house retrofit needs to address together. A properly specified retrofit tackles fuel poverty through fabric and heating measures, manages overheating risk through shading and ventilation, and reduces condensation and mould through a genuine whole-house ventilation strategy – all as one coordinated design, rather than fixing one risk and inadvertently making another worse.

PAS 2035 Retrofit Assessments Retrofit Coordination Services Retrofit Design Retrofit

PAS 2035:2023 Changes

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A Note on the SAP 10 / RdSAP Transition

With RdSAP 2012 still valid ahead of the go-live date for the Home Energy Model, there’s no need to hold off starting assessments now – any EPC lodged under the current version remains usable for PAS purposes, and if no EPC exists yet, an EPC report is accepted in its place. It’s worth factoring into programme timelines regardless, since the transition doesn’t block current delivery but does affect how compliance evidence should be planned going forward.

Coordination Tips to Avoid Delays

Across both the podcast and the masterclass, the same message holds: early planning, high-quality data, and structured coordination are the foundation of successful retrofit delivery. Proactive clients who prepare their data and archetyping strategy ahead of funding being confirmed consistently have smoother projects than those who wait and rush at the last minute. Common delay triggers are avoidable – poor data, late-stage compliance issues and unclear roles are the recurring culprits, not one-off bad luck.

What’s Next?

Both the podcast and the masterclass offer practical technical guidance for teams actively delivering Warm Homes Wave 3 projects. If you’re involved in Warm Homes delivery and want to talk through retrofit delivery, we’d be happy to help.

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If you’d like to discuss technical preparation for your own Warm Homes retrofit programme, get in touch.