Measuring eaves height on a sloping site

On a sloping plot, the height of an extension under Part 1 of the General Permitted Development Order 2015 is measured from the highest part of the surface of the ground next to the building, not from the lowest and not from an average. Measuring eaves height sloping ground extension work therefore produces a smaller figure than most people expect, which usually helps you. The limit that bites is the three metre eaves height where any part of the extension sits within two metres of a boundary of the curtilage.

Get both definitions right and most level disputes disappear. Get either wrong and you have built something outside the class.

Which point the measurement is taken from

Part 1 defines height by reference to ground level, and defines ground level as the level of the surface of the ground immediately adjacent to the building. Where that surface is not uniform, it is the level of the highest part of the surface of the ground adjoining the building.

That wording is doing a lot of work. It means you take one point, the highest adjoining point, and measure vertically from it, rather than measuring from the ground directly beneath each part of the extension. On a plot falling away towards the rear garden, the highest adjoining ground is usually against the original rear wall of the house, so the measurement is taken from there even though the far corner of the extension stands a metre lower.

The consequence is that a rear extension on a falling site can be taller in absolute terms than one on a level site while still complying, because the measured height is smaller. That is a quirk of the drafting rather than a concession, and it applies only to the Part 1 classes.

Three limits in Class A use height and each one takes the measurement the same way: the overall height must not exceed the height of the highest part of the roof of the existing house, a single-storey rear extension must not exceed four metres, and the eaves limit discussed below.

The highest part of the ground next to the building

Which ground counts is the part of the question people argue about. The level is taken from the ground adjoining the building, which in practice means the ground immediately against the walls of the enlargement itself rather than the highest point anywhere in the garden.

So a plot with a bank at the bottom of the garden does not give you a higher datum. A terrace against the back door does, if it is the original ground surface rather than something recently built up.

Two refinements matter on real sites. First, where the extension wraps around a corner, the highest adjoining point may be on a return elevation rather than on the main rear wall, and the measurement still comes from that highest point. Second, where there is a lightwell, a basement entrance or a sunken patio against the wall, that is a lower point and it does not help you, because the rule reaches for the highest rather than the lowest.

An honest uncertainty here. The Order does not define how far from the wall "adjoining" extends, and councils interpret it over a modest distance rather than metres. Where a few hundred millimetres of level change would decide whether a scheme complies, that is a question to put to the council in writing rather than a judgement to make yourself.

The three metre eaves limit within two metres of a boundary

This is the limit that catches householder extensions on sloping plots more than any other. Where any part of the enlargement falls within two metres of a boundary of the curtilage, the eaves height of the enlarged part must not exceed three metres.

Two points on the arithmetic. The two metre test is about the position of the extension, so if any part of it comes within two metres of the boundary, the three metre eaves limit applies to the eaves of the whole enlarged part, not just the part that is close to the boundary. And the eaves height is measured from the same datum as any other height under Part 1, which is the highest adjoining ground.

The height of the eaves means the height of the base of the eaves, which is the point where the external wall meets the underside of the roof rather than the top of the fascia board or the gutter. A flat roof with a parapet raises its own question, because there are no eaves in the ordinary sense and councils generally take the top of the roof structure at the wall line. Put the dimension on the drawing with a leader line to the exact point you have measured to, and the ambiguity stops being your problem.

Outbuildings follow the same logic with tighter numbers, and the interaction between height and the fifty per cent curtilage test is worth reading alongside this, since the Class E curtilage test for outbuildings is often what actually constrains a garden building on a sloping plot.

Split level and stepped plots

Properties built across a slope frequently have a split level plan, with the rear rooms a half storey below the front. The measurement rule copes with that better than the drawings usually do.

Where the extension adjoins only the lower part of the house, the highest adjoining ground is the ground against that lower section, not the ground at the front door. Where the extension steps down with the slope, so that it has two distinct roof levels, each enlarged part is measured from the highest ground adjoining it, which can mean two different datums on the same drawing. Label them separately.

Houses on an upslope site, where the garden rises behind, produce the generous version of the rule: the highest adjoining ground is at the rear, so a single-storey rear extension can be taller at the eaves than the drawings suggest when seen from inside the house while still measuring under three metres.

Roof alterations follow a different set of tests again, and Class B roof extensions and the cubic content allowance use volume rather than height for the main limit, so do not carry the datum reasoning across without checking.

Retaining walls, terracing and made ground

Changing the ground is not a way around the limits, and attempting it creates two new problems.

The first is that regrading, terracing and retaining structures are engineering operations in their own right. A substantial retaining wall or a raised terrace can be development needing planning permission, and raised platforms are excluded from some of the Part 1 classes expressly.

The second is evidential. A council assessing whether an extension complies looks at the ground as it was before the works, and recently imported material against a wall is obvious from the planting, the surface and any photographs. Building up levels and then claiming a higher datum is the sort of argument that fails at the enforcement stage, long after the money has been spent.

Where levels really were altered in the past, perhaps decades ago when a patio was laid, say so and evidence it. That is a reasonable position. Creating the condition so as to comply is not.

What to show on the submitted section drawing

The section drawing is the document that settles this, and most section drawings submitted with householder applications are not good enough.

1. Existing and proposed ground levels, to a datum, taken along the line of the extension and continued beyond the boundary where the neighbouring level differs.
2. A clear marker for the point taken as the highest adjoining ground, with its level stated.
3. The eaves line, with the dimension from that datum to the base of the eaves, and a leader line to the exact point measured.
4. The two metre offset from the boundary drawn on both the plan and the section, so the reader can see whether the limit applies.
5. Overall height to the ridge from the same datum, and the existing house ridge level for comparison.
6. Any retaining structure, with its height and the existing level either side.

A section at 1:50 with four dimensions on it does more for a borderline case than three pages of written justification.

Evidence a council accepts for existing levels

For a straightforward plot, a competent measured survey showing spot levels along the rear elevation and across the proposed footprint is enough, tied to a site datum and cross-referenced to a photograph.

Where the case is close to a limit, or where there is any suggestion that levels have changed, go further. A topographic survey by a surveyor, with levels to Ordnance Datum and a plan showing the survey points. Dated photographs with a levelling staff in shot. Historic drawings, estate agent particulars or building control records showing the previous arrangement. Where a neighbour disputes the position, the survey is what resolves it.

And remember what prior approval and permitted development cannot give you. A Lawful Development Certificate under section 192 of the Town and Country Planning Act 1990 is the only formal way to confirm that proposed works are lawful. On a sloping site where the eaves figure lands within a hundred millimetres of three metres, that certificate is the sensible purchase before the foundations go in, and a short Planning Statement setting out each limit with the measured figure against it is what the application needs.

This is general guidance, not legal advice; 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 your extension 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.