21 December 2023

While new builds are designed to be energy-efficient from the outset, most of the building stock we have existed before modern energy-efficiency home standards were introduced. This is true now and will still be true in 2050. Most UK housing will require significant energy improvement measures to meet the energy efficiency and sustainability levels required by the UK Government’s net zero strategy. To that end, targets have been set for fuel-poor homes to have an EPC of C or above by 2030. Existing buildings must be well insulated and use low-carbon heating and other energy efficiency measures (EEMs).

Retrofitting assets

Retrofitting adds energy efficiency measures to buildings where this wasn’t considered at construction – reducing energy consumption and bills, providing adequate ventilation, increasing thermal comfort and improving occupant health.

  • Whole-building retrofit considers the building as a system of interacting elements, interfaces and occupants, rather than individual components. This approach considers the environment, building use, and occupant desires, while avoiding common retrofit issues found when elements are approached in isolation.
  • The fabric-first approach maximizes a building envelope’s performance by initially reducing heating demand and then introducing low-carbon heating and renewable energy sources. This includes improving the building fabric through repairs – and other simple measures (described by PAS 2035 as ‘low-hanging fruit’) like energy-efficient lighting and improved controls.
  • 'Build tight, ventilate right' is the long-standing battle cry for creating energy-efficient buildings – improving fabric with insulation and airtightness, incorporating suitable ventilation, minimizing thermal bridging to reduce heat loss and lowering heating demand.

Lack of universal standards and a natural retrofit framework

Historically, the lack of universal standards has led to problems with poorly installed EEMs in the form of:

  • Mould growth and condensation.
  • Increased fire risk.
  • Unintended consequences, such as poor health and social problems.
  • Shallow retrofit – making minor, small-scale alterations.
  • Poor or no accountability for completed work.
  • Performance gap – actual performance is much lower than designed targets.
  • Measures considered in isolation rather than as a whole (detrimental to overall building energy efficiency).

The former Department for Business, Energy and Industrial Strategy (BEIS) determined in 2018 that large-scale retrofitting is not viable until such issues are resolved and mandatory new working practices are introduced. In place of a UK Government-produced national retrofit framework, the Construction Leadership Council (CLC) has published Greening Our Existing Homes: National Retrofit Strategy, which aims to:

  • Improve the energy efficiency of existing homes.
  • Improve energy security and resilience of water suppliers.
  • Improve comfort, air quality, lighting, acoustics and occupant safety.
  • Reduce energy bills.
  • Create new, highly skilled jobs.
  • Help meet UK Government carbon emissions and fuel poverty targets.

Each Home Counts

Each Home Counts is an independent report published in 2016 that is based on a 2015 review of consumer advice, protection and standards, relating to home energy efficiency and renewable energy. The review showed that consumers had little protection or guarantee that EEMs were correctly installed or led to savings. The report makes 27 recommendations for consumer protection, most notably:

  • Creating a sector quality mark.
  • Developing a technical standard framework for end-to-end retrofit delivery.
  • Implementing an industry-wide code of practice.

TrustMark, the sector quality mark

TrustMark is the response to ‘Each Home Counts' recommendation for a sector quality mark. The Government-endorsed quality scheme provides a single quality mark for all energy efficiency and renewable energy measures in conjunction with proper oversight. The scheme offers consumers:

  • Reassurance that member companies comply with an operating requirements framework.
  • Confidence of high-quality protected work at an agreed cost via a code of conduct and customer charter.

All public funding applicants must be members of TrustMark organizations. Businesses not using public funding can voluntarily apply for TrustMark to engage with customers who recognize and expect the mark as a prerequisite.

TrustMark's downloadable retrofit guide, Retrofit your Home, gives homeowners an overview of domestic retrofit.

BSI Retrofit Standards Framework for end-to-end retrofit delivery

The British Standards Institution (BSI)’s Retrofit Standards Framework provides the recommended technical standards framework. It comprises three Publicly Available Specifications (PAS). PAS are fast-tracked standards created to meet an immediate need. If a PAS is used to form the basis of a full British Standard, it is withdrawn and replaced by that Standard.

For the Retrofit Standard Framework, the relevant PAS are:

  • PAS 2030:2023: ‘Specification for the installation of energy efficiency measures (EEM) in existing buildings’.
  • PAS 2035:2023: ‘Retrofitting dwellings for improved energy efficiency – Specification and guidance’.
  • PAS 2038:2021: ‘Retrofitting non-domestic buildings for improved energy efficiency’.

All standards referred to in PAS 2030, PAS 2035 and PAS 2038 form part of the Retrofit Standards Framework and require TrustMark users to comply, where applicable. The BSI’s White Paper, The importance of standards for safe energy retrofit, provides an overview.

PAS 2030:2023: ‘Specification for the installation of energy efficiency measures (EEM) in existing buildings’

PAS 2030 sets out requirements for EEM installation, commissioning and handover in domestic retrofit projects and focuses on output quality at all stages. Installers must be PAS 2030-certified when carrying out EEMs under programmes using public funding. Once certified as a PAS 2030 installer, they must register with TrustMark.

PAS 2035:2023 ‘Retrofitting dwellings for improved energy efficiency – Specification and guidance’

PAS 2035 is the framework’s overarching document. Design specifications using the framework must be installed as PAS 2030 and PAS 2035 compliant. This is required for all publicly funded projects and businesses operating under TrustMark.

Key focuses include:

  • Whole-house retrofit.
  • Occupancy comfort, health and well-being.
  • Dwelling functionality, usability and durability.
  • Improved energy efficiency and reduced energy use, carbon emissions and bills.
  • Closing the performance gap.
  • Medium-term improvement plans for all projects.
  • Protection and enhancement of architectural and cultural heritage within the existing housing stock.

Several new roles have been created due to PAS 2035 – each with clear responsibilities, accountabilities, qualifications and accreditation requirements. One person can undertake multiple roles with suitable qualifications (as defined in PAS 2035, annex A). Roles are listed below.

  • Retrofit Assessor: undertakes property assessments and gathers building information (construction, features, structure, condition, etc.).
  • Retrofit Coordinator: provides end-to-end project management with full responsibility for demonstrating compliance and having oversight of installation and handover, working alongside the installer.
  • Retrofit Designer: prepares information packages for retrofit measures, covering EEMs, products, materials, relationships, etc., to be installed to achieve specified energy efficiency.
  • Retrofit Evaluator: monitors and evaluates the effectiveness of the measures, providing feedback to the client and project team – typically post-fit-out. They are also responsible for gathering data and in-depth analysis using control monitors or intrusive surveys.
  • Retrofit Installer: undertakes physical placement of EEMs installed following PAS 2030.

Although PAS 2035 is not required for non-funded projects, it is recommended for deep retrofit residential projects and other projects outside the TrustMark framework:

  • Providing a clear path to compliance and best practices for assessment, design, installation and EEM monitoring.
  • Addressing fundamental problems that affect many domestic retrofits, including defects, unintended health consequences, shallow retrofit, accountability, poor design and performance gaps.

PAS 2030/ 2035 relationship and 2023 updates

PAS 2030 and 2035 are 'locked together'; one may not be used without the other. The only exception is when some measures are installed following Microgeneration Certification Scheme (MCS) standards. (The MCS allows independent certification of renewable energy technologies, microgeneration products and installers as per documented standards.)

The BSI will regularly update all PAS documents within the framework to ensure they remain fit for purpose and deliverable. Following an open consultation in September 2023, PAS 2030 and PAS 2035 were updated to include the following fundamental changes:

The updated publications are freely available and can be downloaded via the BSI’s PAS 2035/2030 web page. There is an 18-month transition period where previous versions remain current until 30 March 2025.

PAS 2038:2021: ‘Retrofitting non-domestic buildings for improved energy efficiency

PAS 2038 serves as the UK Standard for energy efficiency and retrofit of non-domestic buildings. Like PAS 2035, it aims to improve the energy efficiency of existing buildings through reduced energy demand and decarbonizing building services but focuses on commercial buildings rather than domestic ones.

PAS 2038 has resulted in the creation of two critical roles for the industry. Each role has its own qualification and accreditation requirements.

  • Lead Professional: provides end-to-end project management, similar to PAS 2035’s ‘Retrofit Coordinator’.
  • Lead Assessor: undertakes the building assessment, identifying its use, condition and energy performance. Lead Assessors must be qualified non-domestic energy assessors (NDEAs). Retrofit Assessors and Coordinators must also be qualified to this level to carry out work as per PAS 2038.

It is expected that, as PAS 2035, government schemes will require professionals to follow PAS 2038 to access funding. PAS 2038 applies specifically to non-domestic buildings. However, not all need to follow this Standard. Assets less than 500 m2 and have a 'typical' construction type and/ or services can opt to follow PAS 2035 instead.

Traditional and protected buildings

A key focus of PAS 2035 is to protect and enhance architectural and cultural heritage. The recent update to PAS 2035 includes the addition of annex E, which outlines requirements for assessing traditional buildings.

The scope of PAS 2038 also includes historic and traditional buildings and those with special protection. However, unlike PAS 2035, the assessment of significance is carried out following BS 7913, and the building assessment as per BS EN 16883.

For more information on retrofitting historic buildings, see:

Funding schemes and additional resources

Several funding schemes require those delivering EEMs to be TrustMark-registered and PAS 2030-certified. Specifications/ designs must comply with PAS 2035 and be installed as per PAS 2030 to receive funding. These include:

For additional, helpful information on retrofit projects, the PAS retrofit standards and funding schemes, see: