Section 74 of the Highways Act 1980 allows a highway authority to prescribe a frontage line for building, known as a building line, for one or both sides of a highway it maintains, and provides that no new building other than a boundary wall or fence may be erected nearer the centre line of the highway than that building line without the authority’s consent.

What a building line actually is

A building line under section 74 is a line set by the highway authority responsible for a particular road, marking how close to that road new building work is generally allowed to come. It applies to a highway maintainable at the public expense, and the authority can set it for one side of the road, or both, depending on the specific stretch involved. Not every road has a building line prescribed; where none exists, section 74 has nothing to add to the ordinary planning and building control requirements that apply anyway.

The consent requirement

Where a building line has been set, section 74 says no new building, other than a boundary wall or fence, and no permanent excavation below the level of the highway, may be erected or made nearer the centre line of the highway than the building line, except with the authority’s consent. A conservatory extending the frontage of a house toward the road is a new building for this purpose, so if it would sit nearer the road than a prescribed building line, this consent requirement applies on top of, not instead of, planning permission and Building Regulations.

How consent decisions are made

The highway authority can give consent for a specified period and attach conditions it considers appropriate, and any conditions attached bind successors in title, lessees and occupiers of the land affected, not just the person who originally obtained consent. Section 74 also confirms that consent cannot be unreasonably withheld, and that any dispute about whether withholding consent was unreasonable can be determined by the responsible minister, except on highways where the minister is itself the highway authority.

What happens without consent

Building nearer the highway than a prescribed building line without the required consent is an offence under the Act, carrying a fine, with a continuing daily penalty for an ongoing contravention. Where a building line prescription itself causes loss to a property owner, for example by restricting how their land can be used, there is a route to claim compensation, but this has to be pursued within six months of the line being prescribed or of notice of it being given, a considerably shorter window than many other property-related time limits.

How this differs from planning permission

A building line consent is a distinct requirement from planning permission and from Building Regulations approval, administered by the highway authority rather than the local planning authority or building control body, even though in many areas the same council may hold more than one of these functions. Getting planning permission for a conservatory does not itself confirm compliance with a building line restriction if one applies to the property’s road, and the two should be checked separately rather than assumed to be covered by a single approval.

Checking whether a building line applies

Because building lines are not universal and are specific to particular roads where a highway authority has chosen to set one, checking with the relevant highway authority, generally the local council in its highway authority capacity, before finalising the position of a conservatory close to the road frontage is the way to confirm whether this additional consent requirement applies to a specific property, rather than assuming it either does or does not without checking.

FAQs

Does every road have a section 74 building line?

No. A building line only applies where the relevant highway authority has specifically prescribed one for that stretch of road.

Does planning permission for a conservatory also cover a building line consent?

No. These are separate requirements administered under different powers, and both need to be checked where a building line applies.

Can a highway authority refuse consent for any reason?

Section 74 says consent cannot be unreasonably withheld, and a dispute about this can be referred to the responsible minister.

What happens if a conservatory is built nearer the road than a building line without consent?

It is an offence under the Act, carrying a fine, with a further daily penalty for a continuing contravention.

Bottom line

Where a highway authority has prescribed a section 74 building line under the Highways Act 1980, a conservatory built nearer the road than that line needs the authority’s specific consent, separate from planning permission and Building Regulations approval, and building without it is a criminal offence. Consent cannot be unreasonably withheld, and compensation for loss caused by a building line prescription has to be claimed within six months. This is general information, not legal advice on a specific property; the relevant highway authority can confirm whether a building line applies before design work is finalised.

Sources

This guide draws on the following primary sources, current as of 22 September 2026:

• legislation.gov.uk, “Highways Act 1980, section 74: Building lines”

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Compare Conservatories is an independent guide. We may earn a fee from some links; this never affects what we write. This article is general information, not legal or planning advice on any specific property — always confirm requirements directly with your local planning authority.