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Building Within 1 Metre of a Boundary: A 2026 Guide to the Part B Fire Safety Rule

Bhavesh Ramburn

The Part B “within 1 metre” rule

A wall or building constructed within 1 metre of a property boundary must be built as a fire-resisting wall under Approved Document B (Fire safety) of the Building Regulations 2010. This is one of the most important and least-understood Part B rules for householders.

The reason for the rule is to prevent fire spread between adjacent properties. A fire that starts in one property can quickly spread to a neighbouring property if the walls are not fire-resisting. By requiring walls within 1 metre of a boundary to be fire-resisting, the Building Regulations ensure that the fire resistance of the wall is not relied upon to be supplemented by the spacing alone.

The primary statute is the Building Act 1984. The technical standard is set out in Approved Document B — Fire safety, Volume 1: Dwellings (or Volume 2 for buildings other than dwellings).

For the broader context on the Building Regulations regime, see our Building Regulations UK guide and our Building Control in the UK guide.

What fire resistance is required?

The fire resistance required for a wall within 1 metre of a boundary depends on the purpose group of the building and the distance from the boundary, but for a typical dwelling the standard is:

  • Walls within 1 metre of a boundary — must have fire resistance from both sides of at least 60 minutes (REI 60 in European classification terms, or 1 hour in older British classification).
  • Walls 1 metre or more from a boundary — fire resistance is reduced, depending on the actual distance and the size of the wall opening. A wall 1 metre from a boundary may only need 30 minutes’ fire resistance if the unprotected area is small.

The 60-minute fire resistance is achieved through the wall construction. Common methods include:

  • 100mm solid masonry (brick or block) — provides the standard 60-minute fire resistance when properly constructed.
  • Cavity wall with two leaves of masonry — provides enhanced fire resistance depending on the construction.
  • Timber frame with fire-resisting plasterboard — typically 2 layers of 12.5mm Type F (fire-rated) plasterboard on each side of the studwork, providing 60 minutes’ fire resistance.
  • Steel frame with fire-resisting cladding — typically 2 layers of fire-rated plasterboard or a proprietary fire-resisting board system.

The Building Control inspector will check the construction details at the post-installation inspection to confirm that the wall meets the required standard.

The “notional boundary” rule

Where two buildings on the same site are more than 1 metre apart, the Building Regulations allow the LPA to use a “notional boundary” rule. The notional boundary is an imaginary line drawn between the two buildings; if the buildings are within 1 metre of the notional boundary (measured perpendicular to the line), the Part B rules apply as if the boundary were real.

The notional boundary is most commonly used for:

  • Two detached buildings on the same site (e.g. a house and a detached garage more than 1 metre apart but within 2.5 metres of each other).
  • Two buildings under different ownership but close to a real boundary (e.g. a house and a neighbouring outbuilding).
  • Two buildings on a common boundary (e.g. a semi-detached pair or a terraced row).

The notional boundary is treated as a real boundary for the purposes of the Part B rules. The Building Control inspector will advise on the notional boundary in ambiguous cases.

Windows, doors, and other openings in walls within 1m of a boundary

A wall within 1 metre of a boundary must be fire-resisting including any windows, doors, and other openings in it. This is a stricter rule than for a wall further from a boundary, where larger unprotected openings are typically permitted.

For a wall within 1 metre of a boundary:

  • Windows must be fire-resisting glazing (typically integrity-only, 30 or 60 minutes depending on the situation). Ordinary domestic double glazing is not fire-resisting and is not acceptable.
  • Doors must be fire doors with intumescent strips and (usually) a self-closing device.
  • Roof windows and skylights must be similarly fire-resisting if they are within 1 metre of the boundary.
  • Service openings (e.g. boiler flues, vent pipes, electrical conduit) must be fire-stopped where they pass through the wall.

For most ordinary replacement window or door installations, this is unlikely to be a concern — replacement windows in a wall within 1 metre of a boundary will need to be fire-rated, which is a much more expensive specification than ordinary double glazing. The Building Control inspector will advise on the exact requirements for a specific installation.

The interaction with planning permission

The Part B “within 1 metre” rule applies to the construction of the wall and is enforced through the Building Regulations regime. It is separate from the planning-permission regime governed by the Town and Country Planning Act 1990.

A project that triggers the Part B “within 1 metre” rule may also require planning permission, depending on the nature of the work. For example:

  • A new extension close to a boundary will require both planning permission (if it is not Permitted Development) and Building Regulations approval (with the Part B “within 1 metre” rule).
  • A new detached outbuilding within 1 metre of a boundary will require both planning permission and Building Regulations approval.
  • Replacement windows in a wall within 1 metre of a boundary will require Building Regulations approval (and, in some cases, planning permission if the property is in a Conservation Area or the windows are in a listed building).

For a worked example of an LPA-specific planning process, see our Bradford planning portal guide or our Bromsgrove planning portal guide.

The 2.5 metre height limit for Permitted Development outbuildings

A separate but related rule applies under the Town and Country Planning (General Permitted Development) (England) Order 2015 (the GPDO). An outbuilding is Permitted Development only if it is no higher than 2.5 metres if it is within 2 metres of a boundary. This is a planning-permission rule, not a Building Regulations rule, and is enforced by the LPA’s planning enforcement team.

The 2.5 metre / 2 metre rule interacts with the Part B “within 1 metre” rule only in that an outbuilding that is within 1 metre of a boundary (triggering Part B) is also within 2 metres of a boundary (triggering the 2.5m height limit for Permitted Development).

For the broader Permitted Development framework, see our Permitted Development Rights in 2026 guide and our householder planning permission guide.

The interaction with the Building Regulations 10-year rule

A wall that was constructed more than 10 years ago in breach of the Part B “within 1 metre” rule is generally immune from enforcement under the 10-year rule in section 124 of the Building Act 1984. The local authority cannot take enforcement action against an existing non-compliant wall that has been in place for more than 10 years.

The 10-year rule does not apply to new work — any new wall, extension, or outbuilding constructed within 1 metre of a boundary must comply with the current Part B rules, regardless of whether the existing building is non-compliant.

For more on the 10-year rule, see our Building Regulations 10-Year Rule guide.

Next steps and resources

Editorial note. This article is for guidance only and does not constitute regulated advice. The Part B “within 1 metre” rule is technical and the specific fire-resistance requirements depend on the design, location, and use of the building. For project-specific questions, consult a Building Control inspector or a chartered surveyor registered with RICS.