Retrofit means modifying an existing building to improve its energy performance – anything from loft insulation to a full fabric-first overhaul paired with solar PV and a heat pump. Most of the UK’s building stock was built before modern energy efficiency standards existed, and retrofit is one of the most direct ways any building owner can cut both running costs and carbon.
In this blog, you’ll learn:
- The different types of retrofit measures, and how they fit together
- How retrofit is regulated under PAS 2035 and PAS 2038
- The benefits of retrofitting
- What an energy efficient retrofit actually involves, with real examples
Types of Retrofit Measures
Retrofit covers a wide range of measures, and they don’t all pull the same weight:
- Fabric measures: insulation, air tightness, and glazing – these form the foundation, since they reduce how much energy a building needs in the first place.
- Ventilation measures: upgrading extract fans or adding mechanical ventilation with heat recovery – these keep pace with a tighter building envelope, addressing the damp and mould risk a leakier building didn’t have.
- Heating and renewables: air or ground source heat pumps, solar PV – these perform best once fabric and ventilation are addressed first, paired with a building that’s actually capable of making use of them.
That sequencing – fabric, then ventilation, then heating and renewables – is what the rest of this guide comes back to, from how the work is regulated through to what it looks like on a real building.
How is Retrofit Regulated?
Funded retrofit in the UK isn’t self-regulating. DESNZ (the Department for Energy Security and Net Zero) sets the funding policy behind schemes like the Warm Homes: Social Housing Fund, the Warm Homes: Local Grant and the Optimised Retrofit Programme for domestic properties, and the Public Sector Decarbonisation Scheme for non-domestic buildings. TrustMark, the government-endorsed quality scheme, administers compliance on the ground – work delivered under a funded scheme has to be lodged with TrustMark before funding is confirmed or drawn down.
The PAS 2035 and PAS 2038 Frameworks
PAS 2035 and PAS 2038 aren’t regulators themselves – they’re the technical frameworks that funded retrofit work has to follow to meet DESNZ and TrustMark’s requirements.
PAS 2035 governs domestic retrofit: how it’s assessed, coordinated and designed, from initial risk pathway through to installation and evaluation.
Non-domestic and commercial buildings – offices, schools, community halls – fall under PAS 2038 instead. It follows a similar whole-building process, but without a Retrofit Coordinator; a Retrofit Lead Professional oversees the project instead, reflecting the added complexity of commercial building services and occupancy patterns.
Which standard applies depends on the building, not the scale of the works: a single flat retrofit and a 300-unit domestic portfolio both require a PAS 2035 retrofit assessment, while even a small community hall requires a PAS 2038 retrofit assessment, since it’s non-domestic.
When is Compliance Mandatory?
Whether either framework is mandatory depends on how the work is funded.
Funded schemes: retrofit delivered through the publicly funded schemes above must comply with PAS 2035 or PAS 2038 to secure and retain that funding, and be lodged through TrustMark.
Privately funded work: neither framework is a legal requirement, but both are increasingly treated as best-practice guidance:
- Homeowners, landlords and contractors working to PAS 2035 on privately funded projects get the same quality management discipline – proper risk assessment, coordination and evaluation – that funded schemes are held to, without being tied to a specific scheme.
- Green lenders and ESG-linked finance providers use PAS 2038 compliance as evidence a commercial retrofit will deliver real-world carbon reduction.
Why is Building Retrofit Important?
Much of the UK’s building stock predates modern insulation standards, and it shows in the bills. Retrofit brings a genuinely wide set of benefits, both for running costs and for the building itself:
- Lower energy bills: reducing the amount of energy a building needs in the first place cuts costs directly – upgrading insulation, windows and heating systems can mean substantial savings.
- Better comfort and health: insulation and air sealing reduce leaks in the building envelope, keeping temperatures more consistent, cutting draughts, and reducing the damp and mould risk that poor ventilation and ageing fabric can cause.
- Property value and compliance: EPC ratings increasingly carry real financial consequences beyond energy use, from MEES compliance risk on rented properties to lower resale and letting value – a retrofitted building protects against both.
- Better returns on further investment: a heat pump or solar PV array added to a leaky, poorly-insulated building rarely performs as well as the same technology paired with a genuinely retrofitted fabric – fabric-first sequencing matters more than which technology you choose.
What Does an Energy Efficient Retrofit Actually Involve?
An energy efficient retrofit isn’t a single upgrade – it’s a sequence of measures, prioritised in the right order. In practice, that means starting with the building envelope before addressing heating systems and renewables, so the fabric is actually capable of making the most of whatever technology goes in afterwards. Three real projects show how that sequencing plays out on an actual building, not just in theory:
- Diocese of Bath and Wells: full retrofit assessments and RdSAP modelling across a 20-property clergy housing portfolio, sequencing which measures – internal wall insulation, glazing upgrades, solar PV – actually suit each building’s construction and heritage sensitivity, rather than applying a single target uniformly.
- St Werburghs City Farm: diagnostic testing and energy modelling sequenced before any technology decisions, building the evidence base for what low carbon investment would actually be worthwhile.
- St Mungo’s: the same sequencing principle applied at portfolio scale – a fabric-first hierarchy across 300 units in Bristol and London.
What’s next?
Retrofitting doesn’t just make a building more efficient and comfortable – done in the right order, it’s the difference between a heating upgrade that pays for itself and one that doesn’t.
We carry out the retrofit assessment and testing work that identifies exactly where a building is losing energy, the coordination that sequences the fix, and the design that specifies it.