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Building Regulations for Windows: A 2026 Guide to FENSA, Part F, L, K and N

Bhavesh Ramburn

Which parts of the Building Regulations apply to windows?

Window installation triggers a specific combination of Building Regulations Parts, depending on the location and type of window:

  • Part F — Ventilation — replacement windows must incorporate adequate background ventilation (typically trickle vents) to maintain air quality and prevent condensation.
  • Part L — Conservation of fuel and power — replacement windows must meet the current U-value target (typically 1.4 W/m²K or better for the whole window, including the frame).
  • Part K — Protection from falling, impact and collision — windows in certain positions (low-level, beside doors, in bathrooms) must incorporate safety glass or be permanently protected.
  • Part N — Glazing safety — glazing in critical locations (doors, low-level panels, baths) must be safety glass, typically toughened or laminated.
  • Part B — Fire safety — windows in fire-resisting walls and certain other locations may have specific fire-resistance requirements.

For most replacement window installations, Parts F, L, K, and N all apply. Part B applies only in specific circumstances (e.g. windows in a fire-resisting wall between a garage and a house, or windows within 1 metre of a boundary).

The primary statute is the Building Act 1984, and the substantive regulations are the Building Regulations 2010. The technical standards are set out in the Approved Documents.

For the broader context on the Building Regulations regime, see our Building Regulations UK guide. For the application process, see our Building Control Application guide.

FENSA and Competent Person Schemes for window installation

The simplest way to comply with the Building Regulations for replacement windows is to use a Competent Person Scheme member for the installation. The two main Competent Person Schemes for windows are:

  • FENSA (Fenestration Self-Assessment Scheme) — the largest scheme, covering around 8 in 10 replacement window installations in the UK.
  • CERTASS (Certification of Competent Persons for the Fenestration Industry) — a smaller but established scheme.

A FENSA or CERTASS member can self-certify that the replacement windows comply with the relevant Parts of the Building Regulations. The certification is recorded centrally and a Building Regulations Completion Certificate is issued automatically. The homeowner is spared the cost and time of a separate local-authority application.

FENSA-certified installers will issue the homeowner with a FENSA certificate, which is the equivalent of a Building Regulations Completion Certificate for the window installation. The FENSA certificate is the document the buyer’s solicitor will ask for when the property is sold.

If the installer is not registered with FENSA or CERTASS, the homeowner must apply to the local authority for a Building Notice (for a single replacement) or a Full Plans application (for a more complex project), and the work will be inspected by a Building Control inspector.

Part F — Ventilation

Approved Document F sets out the requirements for ventilation. For window installation, the key Part F requirements are:

  • Background ventilation — replacement windows must incorporate background ventilators (typically trickle vents) capable of providing the minimum specified background ventilation rate. The exact rate depends on the room — typically 4000 mm² equivalent area for a habitable room, and 2500 mm² for a kitchen.
  • Extract ventilation — replacement windows in kitchens and bathrooms should not interfere with the operation of the existing extract fan. If the window is the primary source of extract ventilation (older properties), the replacement window must incorporate an equivalent extract facility.
  • Rapid ventilation — windows that are intended to be opened for rapid ventilation (e.g. for summer cooling) must remain openable. Lockable handles are acceptable, but the window must be openable from inside the room.

The Part F requirements are designed to prevent condensation and mould growth, which are common problems in modern airtight homes.

Part L — Conservation of fuel and power

Approved Document L sets out the requirements for the energy efficiency of buildings. For window installation, the key Part L requirements are:

  • Window U-value — replacement windows must meet the current U-value target. The whole-window U-value (the rate of heat loss through the window including the frame, glass, and spacer bar) must be no worse than 1.4 W/m²K for most replacement windows in existing dwellings. The current standard varies depending on the type of property and the part of the building.
  • Air leakage — the installation of the windows must be sufficiently airtight to prevent excessive heat loss. Most modern window installations use foam sealants and pre-compressed tapes around the frame to achieve the required air-tightness.
  • Roof windows and rooflights — replacement roof windows and rooflights have their own U-value targets, which are typically slightly less stringent than those for vertical windows.
  • Draught-proofing — the windows must be installed with appropriate draught-proofing to the perimeter of the frame.

Most modern double-glazed windows exceed the Part L U-value target by a comfortable margin. The U-value target is most likely to be an issue for very large windows or for listed-building applications where the original single-glazed windows must be retained.

Part K — Protection from falling, impact and collision

Approved Document K sets out the requirements for protection from falling, impact, and collision. For window installation, the key Part K requirements are:

  • Low-level glazing — any glazing within 800mm of the floor must be safety glass, typically toughened or laminated glass. This applies to full-height windows, low-level panels in larger windows, and glazed doors.
  • Bathing areas — glazing in bathrooms and shower rooms must be safety glass if it could be reasonably expected to be walked into or fallen against.
  • Door glazing — any glazing in a door, or within 300mm of a door, must be safety glass.
  • Protective barriers — windows installed in a position where someone could fall more than 600mm through them (e.g. a low-level window beside a stairwell) must either be safety glass or be protected by a barrier.

Part K is primarily a safety standard designed to prevent injuries from accidental impact with glazing.

Part N — Glazing safety

Approved Document N sets out the requirements for glazing safety. It is closely related to Part K but is more focused on the type of glass itself. For window installation, the key Part N requirements are:

  • Safety glass classification — safety glass in the UK is classified as either Class A (toughened glass, breaks into small relatively harmless pieces) or Class B (laminated glass, holds together when broken with a plastic interlayer). Class A is more common for windows; Class B is more common for overhead glazing and balustrades.
  • Manifestation of large panes — large panes of clear glazing in critical locations (e.g. full-height windows beside doors) must be made visible by the addition of manifestations — contrasting bands, dots, or logos that make the glass visible to people approaching it.
  • Marking — safety glass must be permanently marked to confirm that it meets the appropriate standard (typically BS EN 12150 for toughened glass or BS EN 14449 for laminated glass).

A competent window installer will specify safety glass for all relevant locations as a matter of routine.

Part B — Fire safety (special cases)

Approved Document B applies to windows in specific circumstances:

  • Windows in fire-resisting walls — where a window is set in a wall that is required to provide fire resistance (e.g. a wall between a garage and a house, or a wall between two dwellings in a converted property), the window must provide a degree of fire resistance. Fire-rated glazing and fire-rated frames are required.
  • Windows within 1 metre of a boundary — in some circumstances, windows within 1 metre of a property boundary may need to be fire-resisting to prevent fire spread to neighbouring properties. The exact requirement depends on the distance from the boundary and the size of the window.

Part B is rarely a concern for ordinary replacement window installations in a single-family dwelling. It is more commonly a concern for flat conversions, HMOs, and commercial premises.

Next steps and resources

Editorial note. This article is for guidance only and does not constitute regulated advice. The Building Regulations requirements for a specific window installation depend on the type, location, and use of the windows. For project-specific questions, consult a FENSA or CERTASS-certified installer, or a Building Control inspector.