Porches: the 3 square metre rule and where it goes wrong
Three numbers decide whether a porch is permitted development. External measurement and the two-metre highway rule remove the rights from most urban front gardens. Here is how to check yours.
Porch permitted development runs on three numbers and nothing else. The ground area must not exceed 3 square metres measured externally, no part may be more than 3 metres above ground level, and no part may be within 2 metres of any boundary of the curtilage with a highway. Meet all three and Class D of the General Permitted Development Order allows a porch outside any external door of a house without an application.
The third number is the one that catches people, and the word "externally" in the first is a close second.
The three limits in Class D
Class D permits the erection or construction of a porch outside any external door of a dwellinghouse. Any external door, so a back door or a side door counts as well as the front.
| Limit | Class D rule |
|---|---|
| Ground area | Not more than 3 m2, measured externally |
| Height | No part more than 3 m above ground level |
| Distance to a highway | No part within 2 m of any boundary of the curtilage with a highway |
There is no restriction on materials, no restriction on roof form, and, unusually for Part 1, no additional restriction for conservation areas or National Landscapes. A porch within Class D in a conservation area is still permitted development, which surprises people who assume every front alteration in a designated area needs consent.
Height is measured from the surface of the ground immediately next to the porch. On a house approached by steps, that is the ground at the base of the structure rather than the threshold level, which usually works in your favour.
Why external measurement changes the answer
Three square metres is a small footprint. A porch 1.5 m wide by 2 m deep is exactly 3 m2 externally, and that is the practical ceiling.
Builders and suppliers frequently quote internal dimensions, because that is what the client cares about standing inside it. Add two 100 mm cavity walls and a 1.5 m by 2 m internal space becomes roughly 1.7 m by 2.2 m externally, which is 3.74 m2 and outside Class D.
Measure to the outer face of the external walls, including any brick plinth, and include the full footprint of the structure rather than the enclosed floor area. Where the porch has a projecting canopy supported on posts, the ground area covered by the structure is what counts.
Fix the dimensions on a drawing before ordering anything, and keep the drawing.
The two-metre highway rule
This is the limb that removes permitted development from most terraced and semi-detached houses on urban streets. The measurement is from any part of the porch to the boundary of the curtilage with a highway, and the highway includes the footway and verge, not just the carriageway.
So a front garden 3 m deep gives you 1 m of usable depth before you are inside the 2 m zone, and a front garden 2 m deep gives you nothing at all. Corner plots lose out twice, because the side boundary is often a highway boundary as well.
Check where the highway boundary actually runs before measuring. It is frequently not the front wall or the hedge. The highway authority holds records of the adopted extent, and on many streets the adopted footway runs a short distance into what householders treat as their garden.
Porches on flats and maisonettes
Class D applies to a dwellinghouse, and for the purposes of Part 1 a dwellinghouse does not include a building containing one or more flats, or a flat within such a building.
That means a ground-floor flat with its own front door has no porch rights at all, however house-like the building looks. The same applies to a maisonette in a converted terrace. A planning application is needed, and the freeholder's consent and the lease terms are separate questions that planning permission does not resolve.
Houses in multiple occupation are dwellinghouses for these purposes where the use falls within class C4, but an Article 4 direction in the area may have removed permitted development rights, so check the council's map.
When a porch becomes an extension
Exceed any Class D limit and the structure is no longer a porch in permitted development terms. The next question is whether it is permitted as an enlargement of the house under Class A, and on a front porch the answer is usually no, because Class A does not permit an extension beyond a wall forming the principal elevation, or beyond a side elevation wall fronting a highway.
A rear or side porch over 3 m2 has better prospects, since it is then assessed against the ordinary rules on depth, height, eaves and curtilage coverage that govern any single-storey extension. Those limits interact with each other, and the rear extension depth limits are the place to start if the porch is at the back.
For a front porch that fails Class D, a householder planning application is the route. Officers assess it against the council's design guidance, and the recurring refusal reasons are the same three every time: a porch that projects far enough to break the building line, a roof form that ignores the rhythm of the terrace, and materials that do not match the house. A short planning statement that addresses those three points directly, with a street elevation showing neighbouring porches, is usually enough on a straightforward case.
Listed buildings and conservation areas
Class D is not disapplied in a conservation area, but two other things can catch you.
An Article 4 direction may have removed householder permitted development rights across the area, and front elevations are exactly what those directions are usually aimed at. Check the council's Article 4 mapping before assuming Class D applies.
A listed building also needs listed building consent for works affecting its special interest, and that requirement is entirely separate from planning permission. Adding a porch to the front of a listed house will almost always affect its character, and carrying out such works without consent is a criminal offence under the Planning (Listed Buildings and Conservation Areas) Act 1990. Where the house is listed or is a curtilage structure to one, expect to need a heritage statement explaining the significance of the building and the effect of the proposal, and take advice before ordering anything.
Building Regulations, and proving lawfulness later
A porch at ground level with a floor area under 30 m2 is generally exempt from the Building Regulations, provided the existing front door between the house and the porch is retained and any glazing meets the safety requirements. Retaining that door matters. Remove it and the porch becomes part of the heated envelope, which brings thermal and other requirements back into play.
When you sell, a buyer's solicitor will ask what authorised the porch. If it was built under Class D, the answer is the Order itself, and the practical evidence is a dated drawing with dimensions and photographs. Where the position or the measurement is arguable, a Lawful Development Certificate is the only formal way to confirm that works are lawful, and it is far cheaper obtained before the porch is built than negotiated during a sale.
This is general guidance, not legal advice. Permitted development rights vary by site and can be removed by condition or Article 4 direction. Check the position with your local planning authority or a qualified professional before relying on it.
Enter your postcode on PlanScope to see which documents a householder application in your area is likely to need. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
How big can a porch be without planning permission?
Not more than 3 square metres in ground area, measured externally, with no part more than 3 metres above ground level and no part within 2 metres of any boundary of the curtilage with a highway. A porch 1.5 m by 2 m externally is exactly at the limit, so measure to the outer face of the walls rather than the internal space.
Does the 3 square metre limit include the walls?
Yes. The ground area is measured externally, so the brickwork, any plinth and the full footprint of the structure all count. An internal space of 1.5 m by 2 m becomes roughly 3.7 square metres once cavity walls are added, which puts it outside Class D even though the usable area is unchanged.
Why does the two-metre highway rule stop so many porches?
Because the measurement runs to the boundary of the curtilage with a highway, and the highway includes the footway and verge. A front garden 3 metres deep leaves only 1 metre of usable depth, and a garden 2 metres deep leaves none. Corner plots are caught twice where the side boundary is also a highway boundary.
Can I add a porch to a flat under permitted development?
No. Class D applies to a dwellinghouse, and for the purposes of Part 1 that does not include a building containing flats or a flat within such a building. A ground-floor flat with its own front door needs planning permission, and the freeholder's consent and the lease terms are separate matters that permission does not resolve.
Do I need consent for a porch in a conservation area?
Class D is not disapplied in conservation areas, so a compliant porch can still be permitted development. But an Article 4 direction may have removed householder rights across the area, and any works to a listed building affecting its special interest need listed building consent, which is a separate regime and a criminal offence to skip.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.