PAS 2035 and the Whole-House Retrofit Approach Explained

PAS 2035 whole house retrofit is the framework behind government-funded home energy upgrades in the UK. If your work is funded through ECO4, the Warm Homes: Local Grant or the Warm Homes: Social Housing Fund, it must follow this standard; the Boiler Upgrade Scheme’s own requirement is an MCS certified installer. Yet most homeowners have never heard of it. This guide explains exactly what PAS 2035 means for you, why it exists, and how the whole-house approach prevents costly mistakes like damp, mould and ventilation failures.

What Is PAS 2035?

PAS 2035 is a Publicly Available Specification published by the British Standards Institution (BSI) in 2019 and updated in 2023. It sets out the process for retrofitting domestic buildings to improve their energy efficiency. Unlike older standards that focused purely on individual measures, PAS 2035 takes a whole-house approach. It requires a proper assessment of the entire property before any work begins.

The standard sits within the broader PAS 2030/2035 framework. PAS 2030 covers the installation of individual measures (the “how”), while PAS 2035 covers the overall retrofit process (the “what” and “why”). Together, they ensure that energy efficiency improvements are planned correctly, installed properly and monitored over time.

Work funded through government energy efficiency schemes must comply with PAS 2035. That includes ECO4, the Warm Homes: Local Grant and the Warm Homes: Social Housing Fund (formerly the Social Housing Decarbonisation Fund), and it included the Great British Insulation Scheme until that closed on 31 March 2026. Private homeowners paying for their own improvements are not required to follow PAS 2035, but the principles remain best practice.

Why the Whole-House Retrofit Approach Matters

Before PAS 2035, energy efficiency measures were often installed in isolation. A home might receive cavity wall insulation one year and new windows the next, with no consideration of how these changes interact. This piecemeal approach caused serious problems.

Insulating walls without addressing ventilation can trap moisture inside the building fabric, leading to condensation, damp and mould. Installing a high-efficiency boiler in a draughty home wastes much of the potential saving. Fitting solar panels on a poorly insulated property means the generated energy is lost through the walls and roof before it can be used effectively.

The whole-house approach treats your home as a system. Every element, from the foundations to the roof, the heating system to the ventilation, the windows to the walls, is considered together. Changes are planned in the correct order so that each improvement builds on the last without creating unintended consequences.

The Fabric-First Philosophy Behind PAS 2035

At the heart of the PAS 2035 whole house retrofit standard is the fabric-first principle. This means improving the building envelope, the walls, roof, floor and windows, before upgrading the heating system. The logic is straightforward: there is no point installing an expensive heat pump if half the warmth escapes through uninsulated walls.

A typical fabric-first sequence might look like this:

• Draught proofing and air tightness improvements
• Loft or roof insulation to at least 300mm mineral wool equivalent
• Wall insulation (cavity, internal or external depending on construction)
• Floor insulation where accessible
• Window and door upgrades to double or triple glazing
• Ventilation improvements to manage moisture
• Heating system upgrade (heat pump, modern boiler or infrared panels)
• Renewable generation such as solar panels

By following this order, the heating system can be sized correctly for the improved building fabric. A well-insulated home needs a smaller, cheaper heat pump. The running costs are lower and the payback period is shorter. For more on heating system options, see our guides to heat pumps and insulation.

The Role of the Retrofit Coordinator

PAS 2035 introduces a central figure: the Retrofit Coordinator. This is a qualified professional who oversees the entire retrofit process from start to finish. They hold a minimum Level 5 qualification in Retrofit Coordination and Risk Management, and they are registered with a TrustMark-approved scheme provider.

The Retrofit Coordinator’s responsibilities include:

• Commissioning and reviewing the initial dwelling assessment
• Identifying risks specific to the property (damp, structural issues, ventilation)
• Producing a medium-term improvement plan, typically a 20 to 30-year roadmap for the home
• Specifying the correct measures in the right installation order
• Selecting appropriate installers with PAS 2030 certification
• Monitoring the quality of work during and after installation
• Ensuring proper handover documentation for the homeowner
• Conducting post-installation evaluation

Every PAS 2035 project needs a Retrofit Coordinator, whatever its risk path; the risk path decides how much design work and oversight the job gets. PAS 2035 isn’t law, but government-funded schemes make it a condition of funding.

The medium-term improvement plan

One of the most distinctive features of PAS 2035 is the medium-term improvement plan. Rather than treating each measure as a one-off project, the Retrofit Coordinator creates a roadmap for the property, typically looking 20 to 30 years ahead. PAS 2035:2023 requires one for every project.

This plan sets out which improvements should be made, in what order, and roughly when. It acknowledges that most homeowners cannot afford to do everything at once. A family might insulate the loft and upgrade the windows this year, add wall insulation in three years when they have saved more, and replace the boiler with a heat pump in five years when the current system reaches end of life.

The plan ensures that work done now does not prevent or complicate work done later. For example, if external wall insulation is planned for the future, the window installation today should account for the increased wall thickness that will come later. Without this forward planning, the windows may need to be refitted at additional cost.

For government-funded projects, the plan is lodged on the TrustMark Data Warehouse. This means that whoever owns the property in the future can access the plan and continue the improvement journey. It also provides an audit trail for quality assurance purposes.

How PAS 2035 Prevents Damp and Ventilation Problems

Damp and mould have been the most visible failures of poorly planned retrofit work, and mould can kill: Awaab Ishak died in 2020 after exposure to mould in his family’s Rochdale social housing flat. Insulation that traps moisture without proper ventilation raises exactly that risk.

PAS 2035 addresses this through several mechanisms:

Moisture risk assessment: Before any insulation work begins, the Retrofit Coordinator must assess how moisture moves through the building. This includes checking for existing damp, understanding the wall construction, and modelling how proposed changes will affect moisture levels.

Ventilation strategy: Any measure that improves airtightness must be accompanied by adequate ventilation. PAS 2035 requires a ventilation assessment and, where necessary, the installation of mechanical ventilation with heat recovery (MVHR) or other appropriate systems.

Correct material specification: The standard ensures that insulation materials are appropriate for the building type. Breathable insulation for solid walls, vapour barriers in the right locations, and materials rated for the specific moisture conditions of the property.

Post-installation monitoring: PAS 2035 requires follow-up checks to verify that the installed measures are performing as expected and that no moisture problems have developed.

PAS 2035 Risk Categories and What They Mean

Under PAS 2035, every retrofit project is classified into one of three risk categories. The risk level determines the level of oversight required.

Most ECO4 and Warm Homes: Local Grant projects involving wall insulation or heating system replacements fall into Path B or Path C. Simple loft insulation top-ups may qualify as Path A. The Retrofit Coordinator decides the path, based on the initial assessment.

Which Government Schemes Require PAS 2035 Compliance?

Most government energy efficiency schemes require PAS 2035 compliance for the measures they fund:

• ECO4: the Energy Company Obligation, funding insulation and heating measures for low-income households until it ends on 31 December 2026
• Great British Insulation Scheme (GBIS): closed on 31 March 2026; it funded single insulation measures for homes in council tax bands A to D
• Warm Homes: Local Grant: council-run grants in England under the Warm Homes Plan, capped at £16,600 for energy efficiency plus £16,600 for low-carbon heating, for homes rated EPC D to G where household income is £36,000 or less, someone gets a means-tested benefit, or the home is in an eligible postcode
• Boiler Upgrade Scheme (BUS): £7,500 towards a heat pump in England and Wales, fitted by an MCS certified installer (PAS 2035 applies where the heat pump is part of a wider funded retrofit)
• Warm Homes: Social Housing Fund: formerly the Social Housing Decarbonisation Fund, funding whole-house retrofit of social housing in England towards EPC C
• Home Upgrade Grant (HUG): replaced by the Warm Homes: Local Grant, which covers homes on and off the gas grid

If you are funding improvements privately, PAS 2035 is not mandatory. However, following its principles protects your investment and makes the work safer. To price a project, request a free quote: up to three approved installers who cover your area may contact you. No charge, no obligation.

How Much Does PAS 2035 Compliance Add to Costs?

The additional cost of PAS 2035 compliance varies depending on the complexity of the project. For a straightforward Path B project, you can expect:

• Retrofit Assessment: £150 to £400 depending on property size and complexity
• Retrofit Coordination: £300 to £800 for the coordination service across the project
• Ventilation assessment: Often included in the retrofit assessment or £100 to £200 separately
• Lodgement fees: £30 to £60 for TrustMark Data Warehouse registration

For government-funded work, these costs are typically included in the funding and are not charged to the homeowner. For private work, the additional cost of £500 to £1,500 is a small price for the protection it provides against costly mistakes. A failed solid wall insulation job can cost £10,000 to £20,000 to remediate.

Frequently Asked Questions About PAS 2035

Do I need to follow PAS 2035 if I am paying for my own home improvements?

No. PAS 2035 is only mandatory for work funded through government energy efficiency schemes. However, the principles of the whole-house approach are strongly recommended for any retrofit project. Following PAS 2035 voluntarily ensures your improvements are safe, effective and properly coordinated. Many reputable installers now follow PAS 2035 processes as standard practice.

What happens if my installer does not follow PAS 2035?

If the work is government-funded and the installer fails to comply with PAS 2035, you have grounds for complaint through the TrustMark dispute resolution process. The installer risks losing their TrustMark registration, which would prevent them from carrying out future funded work. For privately funded work, non-compliance is not a legal issue but may affect insurance claims if problems arise later.

Can I choose which measures to install or does PAS 2035 decide for me?

The Retrofit Coordinator recommends the most appropriate measures for your home, but you retain the final decision on what is installed. The coordinator will explain the risks of installing certain measures without others. For example, they may advise that wall insulation should not proceed without adequate ventilation. You can choose to defer measures to a later date as part of the medium-term improvement plan.

How long does the PAS 2035 process take from assessment to installation?

For a straightforward single-measure project, the process from initial assessment to completed installation typically takes four to eight weeks. Complex whole-house retrofits involving multiple measures may take three to six months. The assessment itself usually takes one to two hours on-site, with the improvement plan produced within two weeks. Government-funded schemes may have additional waiting times due to demand.

Is PAS 2035 the same across England, Scotland and Wales?

PAS 2035 is a UK-wide standard published by BSI. It applies equally in England, Scotland and Wales. However, the government schemes that require PAS 2035 compliance differ by nation, and Scotland and Wales run their own programmes, such as Warmer Homes Scotland and Nest, each with its own installer requirements. The technical requirements of the standard are identical regardless of location. To see which schemes may fit your home, try our grant checker.