Eaves height limits when an extension sits near a boundary

Two numbers govern extension eaves height near a boundary. The eaves of a single storey extension cannot be higher than the eaves of the house it is attached to, and where any part of the extension comes within 2 metres of the boundary of your curtilage, the eaves cannot exceed 3 metres. Both come from Class A of Part 1 of Schedule 2 to the GPDO 2015. Miss either and the extension is not permitted development, however modest it looks from the garden.

The 3 metre figure is the one that catches people. It is not a rounding of the general 4 metre height limit, and it applies to the eaves rather than the ridge.

The general eaves height limit

Class A does not give a flat number for eaves on every extension. The default control is comparative: the eaves of the enlarged part must not be higher than the eaves of the existing dwellinghouse. On a standard two storey house with eaves at around 5 metres, that is not a constraint anyone bumps into with a single storey rear extension.

Where it does bite is on bungalows and chalet bungalows with low eaves, and on houses that have already been extended so that the reference point is a previous flat roof. The comparison is with the existing house, not with the original one, which is unusual in Class A where most tests refer back to the original dwellinghouse as it stood on 1 July 1948 or as first built.

Separately, a single storey rear extension must not exceed 4 metres in height overall, and a side extension must not exceed 4 metres. Those are ridge or parapet limits, and they sit alongside the eaves rules rather than replacing them.

The reduced limit within two metres of a boundary

The provision that matters is A.1(i). If the enlarged part would be within 2 metres of the boundary of the curtilage of the dwellinghouse, and the eaves height of the enlarged part would exceed 3 metres, permitted development does not apply.

Three details decide most cases.

1. The 2 metres is measured horizontally from any part of the enlarged part to the boundary, at the closest point. A wall that runs at a slight angle can be inside 2 metres at one end and outside it at the other, and the whole extension then takes the 3 metre limit.
2. The boundary is the boundary of your curtilage, not the neighbour's fence. Where a fence sits inside the legal line, or where a strip of your land runs behind a garage, the relevant line may be further away than it looks.
3. There is no allowance for the eaves being set back above a lower wall. The test reads on eaves height, full stop.

A rear extension pushed hard against a side boundary, which is the standard suburban arrangement, will almost always fall under this rule. Designing a mono-pitch roof that starts at 2.9 metres against the fence and rises away from it is the usual answer. On a wrap-around, the same limit applies to the side leg, and the interaction between the rear and side limits is where most schemes lose their permitted development status.

Measuring from the natural ground level

Height under Part 1 is measured from the highest part of the ground next to the building, as the government's technical guidance for householder permitted development explains. That single rule resolves a surprising number of arguments.

The eaves height itself is taken to the point where the external wall would meet the upper surface of the roof if the wall were extended upwards. So the fascia board and the gutter hanging below that line do not set the figure, and a deep fascia does not push a compliant extension over the limit.

What does change the figure is the ground. Because the measurement starts from the highest adjacent ground rather than the lowest, a garden that falls away from the house gives you a measured height smaller than the one a tape from the patio would show.

Where a sloping site changes the answer

On a site that slopes across the rear elevation, the extension has one eaves line and two ground levels. Measured from the high end, the eaves may be 2.8 metres. Measured from the low end, 3.3 metres. The rule points at the high end, so the scheme is compliant, but only if the drawings say so clearly.

That is the practical problem. A case officer looking at a validation drawing with no levels on it has no way to apply the rule, and the usual outcome is a request for further information or an invalid application. Put a spot level on the existing ground at each corner of the extension, tie them to an assumed datum, and dimension the eaves from the relevant one.

Ground reprofiling is the other trap. Raising the ground next to a wall does not lawfully raise your allowance, because the measurement is taken from the ground as it was before the development, and an officer who sees imported material against a new wall will treat the pre-existing level as the starting point.

Parapets, fascias and what counts as eaves

Flat roof extensions are where the definition gets thin. A flat roof has no eaves in the pitched sense, and Class A does not define the term.

In practice most authorities take the eaves height of a flat roof extension as the height of the top of the external wall where it meets the roof structure. A parapet standing above that is then treated as part of the roof rather than as eaves, which is the applicant-friendly reading, and it is not universal. A parapet detail near a boundary is therefore one of the few householder questions worth resolving formally before building.

A lawful development certificate under section 192 of the Town and Country Planning Act 1990 is the only way to confirm that proposed works are lawful. It costs a fraction of a householder application and it is the document a buyer's solicitor will ask for in eight years' time.

Flat roof extensions and overall height

The 4 metre overall limit on a single storey rear extension and the 3 metre eaves limit near a boundary work together to produce a familiar shape: a flat or shallow mono-pitch roof against the boundary, rising to a higher flat roof or a rooflight zone away from it.

Watch the build-up. A 150 millimetre firring, 120 millimetres of insulation, the deck and a single ply membrane add up faster than the sketch suggests, and the parapet upstand sits on top of that. Extensions that were drawn at 2.95 metres are routinely built at 3.05 metres. Roof plant and flue terminations are a separate matter, and a stove flue rising above the new roof has its own limits.

Designing to the limit safely

Leave yourself 100 millimetres. The rules are clear enough, but brick courses, screed depths and site levels all move on site.

Illustrative example: a 1930s semi with a rear garden falling 400 millimetres over the depth of a proposed 3 metre extension, built tight to the side boundary. Eaves set at 2.85 metres above the highest adjacent ground, overall height 3.6 metres to a shallow pitch, spot levels on all four corners of the extension shown on the section. Nothing exotic, and it stays inside Class A because the drawings prove it does rather than assert it.

Where the eaves have to exceed 3 metres near the boundary, the fallback is a householder planning application. The case then turns on daylight, outlook and the 45 degree test rather than on the Class A arithmetic, and a short planning statement dealing with the neighbour's windows is worth the effort.

Enter your postcode on PlanScope to see which supporting documents a householder extension in your area is likely to need.

This is general guidance, not legal advice, and a lawful development certificate is the only formal way to confirm that works are lawful. Check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.