Building Regulations in Wales: Where Welsh Rules Differ

Building regulations are the rules that decide whether the work on your home is safe, healthy and energy-efficient, and in Wales they are a devolved matter: the Welsh Ministers, not the UK Government, set the standards and approve the guidance. The legal backbone is still the Building Regulations 2010, which apply to Wales under regulation 2C¹, but they have been amended repeatedly by Wales-specific instruments, most significantly in 2022 and again in 2026². The regulations control certain types of building work and ensure buildings meet standards of health, safety, welfare, convenience and sustainability³.

For a householder, the practical questions are simple: does a project count as "building work", does anyone need to be told before it starts, and who carries responsibility if it is done badly? The Welsh answers differ from England's in ways that matter, particularly on ventilation, where Welsh rules on background ventilators are more demanding than the English equivalents. This page sets out the Welsh system as it stands, what counts as notifiable work, and what evidence an owner holds at the end.

What the Welsh rules are for

The Building Regulations exist to make sure that buildings, and the work done to them, meet minimum standards. The Welsh Government describes their function plainly:

In setting those standards, the regulations also influence emission reductions in both new and existing buildings in Wales⁹.

The system works through two layers:

Following the guidance is the usual route to compliance, but the legal duty is to satisfy the regulations themselves.

For a household planning work, the first step is establishing whether the project is "building work" as defined in Regulation 3 of the Building Regulations⁷. If it is, the work must either be notified to a building control body in advance or be carried out by an installer who can self-certify it under a competent person scheme.

How Wales differs from England, Scotland and Northern Ireland

Building regulations are devolved, and each nation runs its own system. England uses the Building Regulations 2010 with its own Approved Documents; Scotland uses building standards and the building warrant system; Northern Ireland uses its own regulations and Technical Booklets. Wales shares the 2010 Regulations with England as a starting point, but the Welsh Ministers have amended them separately since devolution, so the standards and the Approved Documents have diverged.

The divergence is most visible in the guidance itself. Planning Portal guidance for Wales repeatedly warns that where its material relates to the English regime, "the policy in Wales may differ" and that householders in doubt should contact their local planning authority¹⁰. The Welsh Approved Documents are published separately: Approved Document L Volume 1 (2026 edition) states that it "gives guidance for compliance with the Building Regulations for building work carried out in Wal[es]"⁶, and the Approved Document L2B consultation material confirms that the documents were "approved and issued by Welsh Ministers"¹¹.

The 2022 Welsh amendments are a good illustration of the divergence. They introduced:

• new energy performance metrics
• mechanical ventilation air flow rate testing on existing dwellings undergoing specified notifiable building work
• overheating mitigation

None of which mirrors the English package exactly². Wales also has its own planning context for energy work: guidance on solar panels, heat pumps, flues and electrical work all note that Welsh policy may differ from England's¹⁰.

There are also classes of building where Wales applies special considerations. Guidance for work in Wales identifies three classes of non-exempt existing buildings where special considerations apply¹³:

• buildings of architectural and historic interest referred to in a local development plan
• buildings of architectural and historic interest within national parks, areas of outstanding natural beauty, registered historic parks and gardens, registered battlefields, the curtilages of scheduled ancient monuments and world heritage sites
• buildings of traditional construction with permeable fabric that absorbs and readily allows the evaporation of moisture

If your home falls into one of these classes, the energy-efficiency standards are applied with that context in mind.

What qualifies as building work

Regulation 3 defines building work, and the definition is broader than many householders expect. It covers⁷:

• the erection or extension of a building
• the installation or extension of a controlled service or fitting
• alteration projects that affect ongoing compliance with the requirements relating to structure, fire, or access to and use of buildings
• the insertion of insulation into a cavity wall
• the underpinning of foundations
• work affecting the thermal elements, energy status or energy performance of a building

In practice, the regulations probably apply if you want to put up a new building, extend or alter an existing one, or provide services and fittings in a building such as washing and sanitary facilities, hot water cylinders, foul water and rainwater drainage, replacement windows, and fuel burning appliances of any type⁷. Extensions bring their own set of requirements, including drainage¹⁵. The regulations may also apply to certain changes of use of an existing building, because the change may result in the building no longer complying with the requirements that apply to its new type of use⁷.

Two energy-relevant points are worth noting. First, insulation has to comply with the relevant building regulations both when installed during construction and when fitted retrospectively¹⁶. Second, the energy efficiency requirements in Regulation 21 apply to the erection or extension of, and any work to or in connection with, a building that is a roofed construction having walls, uses energy to condition the indoor climate, and does not fall within an excluded category¹⁷. That reaches deep into ordinary retrofit work: cavity wall insulation, window replacement and heating upgrades are all within the system.

A condition runs through all work on existing buildings: the works must not make other fabric, services and fittings less compliant than they were before, or dangerous⁷. Replacement double-glazing, for example, must not make compliance worse in relation to means of escape, air supply for combustion appliances and their flues, and ventilation for health⁷.

Cases where approval is unnecessary: exemptions, repairs and self-certification

Not everything requires an application. Some kinds of building projects are exempt from the regulations altogether¹⁴. Historic and traditional buildings have their own exemptions: the 2014 Welsh Approved Document L1B lists listed buildings, buildings in conservation areas and scheduled monuments among classes exempt where compliance would unacceptably alter their character or appearance¹⁸. Conservatories and porches are exempt if thermally separated from the building and containing no fixed heating appliance, or where the building's heating system is not extended into them; the exemption ceases if the separating walls, windows or doors are removed, or a fixed heating appliance is provided¹⁹.

Even where the regulations apply, there are two routes that avoid a full application. The first is the competent person scheme: you do not need to get approval yourself if you use someone registered with a scheme, because they self-certify and notify on your behalf⁸. The second is the category of non-notifiable work. For electrical work in England and Wales, the regulations identify non-notifiable work as²⁰:

• any repair or maintenance work or like-for-like replacement
• any addition or alteration to an existing circuit that is not in a kitchen, bathroom or outdoors
• installing or upgrading main or supplementary equipotential bonding
• installing extra low voltage cabling outside bathrooms for telephone, fire alarm, burglar alarm and heating systems

The same principle appears in the general guidance: repairs, replacements and maintenance work, and extra power or lighting points or alterations to existing circuits, except in a kitchen or bathroom or outdoors, do not need to be reported²¹.

Specific projects have specific answers:

Welsh Part L: energy requirements for new and existing homes

Part L is the part of the regulations that deals with the conservation of fuel and power, and it is where Wales has moved furthest from the English baseline. The 2022 amendment regulations, made on 20 May 2022, apply only to buildings and building work in Wales and introduced new ways of measuring energy efficiency using new performance metrics². Approved Document L Volume 1, in its 2026 edition, provides practical guidance on ways of complying with the energy efficiency requirements of the Building Regulations 2010 for Wales⁶.

The results are visible in the statistics. The 2022 amendments to the Building Regulations require a minimum Energy Efficiency Rating of B for new dwellings in Wales, and official statistics note that the increase in the proportion of new dwellings rated band A may be a result of those amendments⁴. In the quarter April to June 2025, 90% of new domestic properties in Wales received an energy efficiency rating of A or B, with 9% rated C or D⁴.

The policy changes behind those numbers were set out in the Welsh impact assessment²⁴:

• improved fabric
• photovoltaics
• wastewater heat recovery
• air tightness tests for all new homes
• the removal of fuel factors
• self-regulating devices in all new homes through changes to the notional specification

The same document records the expected benefit: reduced energy costs of £213.4m present value for new dwellings, with air tightness testing contributing energy savings of £0.1m²⁴. The changes sit under Policy 45 of Net Zero Wales, which commits Wales to set higher energy efficiency standards for new builds through reviewing Part L²⁴.

The 2022 package also introduced Part O, on overheating: dwellings shall be designed and constructed to provide reasonable mitigation from the risk of summertime overheating, with mitigation measures that are safe, secure and reasonably practical for occupants²⁴, and sufficient information about the Part O provisions must be given to the owner². For heat pumps, the Welsh consultation material sets a minimum coefficient of performance of 2.0 for heating domestic hot water with other types of heat pump¹⁹.

For existing homes, the standards work differently. The 2014 Approved Document L1B for existing dwellings in Wales set limiting U-values including 0.35 W/m2.K for roofs under the optional approach¹⁸, and the consultation material for retained windows and doors gives a threshold U-value of 3.3 W/(m2.K) alongside options of 1.4 or Window Energy Rating Band B, or 1.2 centre pane or low-e glazing¹⁹. The governing principle is the non-regression rule: work must not leave the building less compliant than before⁷.

Transitional arrangements matter when standards change. The 2022 amendments do not apply where the building work has already commenced or is started within 12 months of the regulations coming into force, subject to a notification condition², and the No. 2 Regulations 2022 similarly require that a building notice or initial notice has been given, or full plans deposited, before the regulations come into force, with work started within 12 months²⁵. Where an application covers a number of buildings on a site, only those individual buildings for which work is commenced can take advantage of the transitional provisions²⁵.

Approved Document F in Wales: ventilation and background ventilators

Ventilation is where Welsh rules are most distinctively stricter. The 2022 amendment regulations require that when Part L work is carried out, the ventilation in the building must comply with the standard set out in Part F of Schedule 1, or it must be no worse than before the Part L works². The logic is simple: insulation and airtightness improvements reduce the air leaking through the fabric, so the ventilation has to be made up deliberately.

The Welsh consultation on Part F proposed increasing the size of background ventilators in new dwellings with mechanical extract ventilation from 2500mm2 to 5000mm2 equivalent area in habitable rooms²⁴. The guidance sets minimum equivalent areas by room type⁵:

An example natural ventilation system for an existing dwelling in the impact assessment comprises 12 background ventilators of 5000mm2 each, a 15 l/s intermittent fan in the bathroom and a 30 l/s intermittent fan in the utility room²⁴.

Replacement windows have their own rule. Where the original window had background ventilators, the new ventilators must not be smaller than those in the original window and must be controllable, either automatically or by the occupant⁵. Where the size of the original ventilators is unknown, the minimum is 8,000 mm2 equivalent area in habitable rooms⁵. Where a new habitable room is added, the guidance calls for background ventilators of at least 10,000 mm2 equivalent area between the two rooms and at least 10,000 mm2 between the additional room and outside, where the existing habitable room has at least 5000 mm2 total background ventilator equivalent area⁵.

Intermittent extract fans in a room with no openable window need controls which continue to operate the fan for a minimum of 15 minutes after the room is vacated⁵. All fixed mechanical ventilation systems, including intermittent extract fans and cooker hoods, for which testing and adjustment is possible, must be tested and commissioned when installed as part of building work⁵. And the person carrying out the work must, not later than five days after the work has been completed, give the owner sufficient information about the ventilation system⁵; the same five-day deadline applies to information about systems for on-site generation of electricity²⁶.

Retrofit work and the ventilation outcome classes

When energy-efficiency work is done on an existing home, Welsh guidance sorts the likely ventilation outcome into three categories⁵:

• Category A: it is likely that the energy efficiency measures have not reduced the ventilation provision of the dwelling below the requirements, and no further ventilation is required.
• Category C: it is likely that the ventilation provision of the dwelling has been reduced significantly below the requirements of F1(1), and expert advice is required for natural ventilation.

The category that applies depends on the work, and the stakes are real. A home that is tightened up without compensating ventilation risks moisture, mould and poor indoor air quality, which is why the 2022 regulations require mechanical ventilation air flow rate testing on existing dwellings which undergo notifiable building work². The underlying rule for all existing-work ventilation is that the building, including the provision of ventilation, should not be made less satisfactory than before the work was carried out⁵.

In kitchens and bathrooms the requirement is explicit: any existing fans, including cooker hoods where they extract to outside, or passive stack ventilators, should be retained or replaced when building work is done in those rooms⁵. The non-regression rule applies to the whole building as well: works must not make other fabric, services and fittings less compliant than they were before, or dangerous⁷.

Who bears responsibility for non-compliant work

Responsibility is shared, and the split matters. People who are responsible for building work, including agents, designers, builders or installers, must ensure that the work complies with all applicable Building Regulations requirements²⁷. If you are employing a builder, the responsibility will usually be theirs, but this should be confirmed at the very beginning of the project⁷. If you are the owner of the building, it is ultimately you who may be served with an enforcement notice if the work does not comply with the regulations⁷.

For heating systems the rule is more specific: if an existing system has been altered or replaced, the person who last worked on the system is responsible for its safe running and should issue a certificate showing the necessary checks have been carried out²³. For energy devices such as solar installations, guidance for England and Wales states that the installation contractor has a responsibility to ensure the work complies with building regulations, and the device owner may also have a responsibility and could be served with an enforcement notice in cases of non-compliance²⁸. Where an installer is not registered with a Competent Person Scheme, the building regulations certificate should be provided within 30 days of completion²⁸.

How rules are enforced: notices, fines and prosecution

If building work does not comply with the Building Regulations, the building owner may be served with an enforcement notice⁵. Beyond that, the person doing the work could be prosecuted and fined if they do not comply with building regulations⁸. The enforcement system also reaches forward into new applications: under the Building (Registered Building Control Approvers etc.) (Wales) Regulations 2024, a person submitting an Initial Notice must inform the local authority about a previous unremedied contravention notice and give details of the contravention²⁹.

Separate penalty regimes sit alongside the building regulations. The Energy Performance of Buildings (Certificates and Inspections) (England and Wales) Regulations 2007, which extend to England and Wales and cover all buildings including buildings exempt from building regulations, set a penalty charge of £200 where the building is a dwelling for breaches of the duty to provide an energy performance certificate in the specified cases³⁰.

The practical protection for a householder is paperwork: a completion certificate from the building control body, or a certificate from a competent person installer. Without it, unauthorised work surfaces later, at sale or remortgage, when there is no evidence the work was ever approved.

The future direction of Welsh regulations

The Welsh system is still moving. The Building etc. (Amendment) (Wales) Regulations 2026 apply to buildings and building work in Wales, and their transitional provisions state that building work must have already commenced or be started within 12 months of the regulations coming into force to fall under the old standards, and that the changes will not apply to work where building control approval has been sought before 1 July 2026²⁹. A household with a project in the pipeline therefore needs to know which version of the standards applies.

The direction of travel is set by Net Zero Wales Policy 45, which commits Wales to set higher energy efficiency standards for new builds through reviewing Part L²⁴, and by the Welsh Government's building regulations sustainability review, which frames the regulations' purpose in terms of health, safety, welfare, convenience and sustainability³. For existing homes, the Welsh Housing Quality Standard requires all existing social homes to meet the standard, with local authorities required to comply pursuant to section 111 of the Housing (Wales) Act 2014³¹, which pulls the social housing stock toward the same energy standards ahead of the private stock.

Implications for a household's independence

The Welsh building regulations cut both ways for a household trying to make its home more self-sustaining. On the one hand, they are the gate through which every energy measure passes: solar panels, flues, insulation, heat pumps, replacement windows and heating controls all engage the system, and the 2022 standards push new homes hard toward low energy use, with 90% of new Welsh homes rated A or B in the second quarter of 2025⁴. A home built or retrofitted to those standards needs less imported energy year on year.

On the other hand, the rules create dependencies of their own:

• Work must be notified, inspected or self-certified, which means relying on a building control body or on an installer's registration with a competent person scheme⁸.
• The five-day information requirement for ventilation systems and on-site generation⁵ means the household's knowledge of its own systems depends on the installer discharging that duty.
• The standards themselves change on the Welsh Government's timetable, not the householder's: transitional windows of 12 months² and a cut-off of 1 July 2026 for the 2026 changes²⁹ mean that a project delayed can become a project built to different rules.

A household that keeps its completion certificates, its ventilation information and its installer certificates holds the evidence that its independence was built lawfully.