Measuring height where the ground slopes
The datum is the highest part of the ground immediately next to the building, so the downhill corner is what breaches the limit. How to measure it and record it.
On a sloping plot, measuring height for permitted development is done from the highest part of the ground next to the building, which means the lowest corner is usually the one that busts the limit. Part 1 of Schedule 2 to the Town and Country Planning (General Permitted Development) (England) Order 2015 defines ground level as the surface of the ground immediately adjacent to the building or extension, and where that surface is not uniform, the level of its highest part. Read it the other way round and you get the common and expensive mistake: measuring from the high side, finding 2.4 metres, and building an outbuilding that is 2.9 metres above the ground at its downhill corner.
The measurement rule in the Order
The interpretation provisions in Part 1 do two things. They fix the datum as the ground immediately adjacent to the building rather than some notional site level, and they resolve non-uniform ground by taking the highest part.
Both halves matter.
Immediately adjacent means right next to the wall in question, not across the garden and not at the site entrance. A terrace that falls away three metres over the plot has a different datum at each elevation.
Highest part sounds generous and is not, because the height limit then applies at every point of the building from that single datum. If the ground next to your proposed outbuilding is 1.9 metres at the top of a slope and the roof is 2.4 metres above that point, the building complies on height. If the roof is set level and the ground drops half a metre to the far corner, the overall height from the lowest adjacent ground is 2.9 metres, and that is what a neighbour photographs and what an enforcement officer measures.
So the practical reading is this: the datum comes from the highest adjacent ground, and the building has to sit within the limit measured from that datum across its whole extent. A flat-roofed outbuilding on a slope therefore needs stepping, or a lower ridge, or it needs permission.
Finding the highest part of the adjacent ground
Do this with a level and a tape, not by eye. Gradients of one in twenty are hard to see and easy to measure.
- Mark the footprint of the proposed building on the ground with pegs at each corner.
- Take a level reading at each corner and at the mid-point of each elevation, immediately against the proposed wall line.
- Record the readings relative to a fixed temporary benchmark that will survive the works, such as a manhole cover or a step.
- Identify the highest of those adjacent readings. That is your datum.
- Calculate the permitted maximum height at the datum, then check the resulting roof level against the ground at every other point.
- Keep the survey. A dated levels drawing is the evidence if the position is questioned later.
Where the ground is irregular rather than evenly sloping, for example a mound of spoil left from earlier works, take enough readings to describe the surface rather than two and an assumption. And consider whether the irregular surface is itself the original ground.
The same datum question drives a great many permitted development disputes, and the detail of how the adjacent ground rule applies under Part 1 is set out in measuring height from adjacent ground.
Within two metres of the boundary
Several of the limits in Part 1 change inside two metres of the curtilage boundary, and that is where sloping ground causes most of the trouble.
For outbuildings under Class E the overall height limit falls to 2.5 metres anywhere within two metres of the boundary. For rear extensions under Class A the eaves height within two metres of the boundary is limited to three metres. Both limits are measured from the same ground level datum.
Two metres is measured to the boundary of the curtilage, which is not always the fence. Where the fence sits inside the legal boundary, or where a strip beyond it belongs to the property, the two metre line moves. Check the title plan rather than the fence line.
A building sited 2.1 metres from the boundary and a building sited 1.9 metres from it are governed by different limits, so the siting decision and the height decision have to be taken together. Pushing a garden room 300 millimetres further from the fence is often what makes a design work.
Eaves height against overall height
These are separate limits and they are measured differently, which is a frequent source of confusion on a slope.
Overall height is to the highest point of the building, excluding chimneys, flues and antennas where the Order allows. Eaves height is to the base of the eaves, meaning the point where the roof plane meets the external wall, and the Order treats the eaves height on a part of a building as the height of the base of the eaves on that part.
On a sloping site a single-pitch roof falling with the slope can satisfy the eaves limit on one elevation and breach it on another. A dual-pitched roof set level will have a consistent eaves height in absolute terms and a varying one relative to the ground.
Check each elevation separately. A compliance statement that gives one eaves height for a building on a slope has not done the work.
Excavated and built-up ground
This is the part where certainty runs out, so treat it carefully.
Where ground is excavated to set a building lower, the argument is that the adjacent ground is now the excavated surface, which lowers the datum and reduces the permitted height. Where ground is built up, the datum rises. Authorities and inspectors have reached different conclusions on variants of this, and the outcome tends to turn on whether the ground works were part of the same operation as the building and whether the altered surface really is the ground immediately adjacent.
Two practical consequences.
Raising ground levels to gain height is unlikely to work and may itself be development. Engineering operations to regrade a garden can require permission in their own right, particularly where retaining structures are involved, and permitted development rights for hard surfaces and for other alterations do not cover wholesale reprofiling.
Excavating to keep a building within the limit does work, but it has to be real: a trench around the walls is not the adjacent ground, and a sunken building needs drainage, retaining structures and a considered relationship with neighbouring land.
Where there is any doubt, 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. It takes eight weeks and a fee, and it is the cost of certainty.
Retaining walls and platforms
A retaining wall built to create a level platform is a structure, and the platform changes the ground.
Decking and raised platforms have their own treatment in the Order, and a platform more than 0.3 metres high is treated as a raised platform, which affects what is permitted in a rear garden and within two metres of a boundary. A building standing on a raised platform has its height measured in a way that captures the platform, so building up and then placing an outbuilding on top does not reset the datum.
Boundary retaining structures bring the neighbour into it as well. Works affecting a shared boundary or excavating close to a neighbour's foundations can engage the Party Wall etc. Act 1996, which runs separately from planning and does not care about permitted development rights.
Recording levels on a survey
Whatever you build, document the starting position before anything is disturbed.
A measured topographical survey showing existing levels across the plot and against each proposed elevation is the single most useful document on a sloping site. Add dated photographs with a levelling staff visible, the title plan showing the legal boundary, and a short note of the datum used and the resulting maximum heights.
Enforcement questions on sloping sites usually arrive two or three years later, from a new neighbour, and by then the ground has been landscaped and nobody remembers what it looked like. The time limits and the process are covered in enforcement time limits for permitted development breaches.
One more check before you start: confirm that permitted development rights apply at all. An Article 4 direction, a condition on an earlier permission, or the property being a flat rather than a house removes them, and the way authorities make those directions is explained in how a council makes an Article 4 direction.
This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. A Lawful Development Certificate is the only formal way to confirm that works are lawful.
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Frequently asked questions
Where is height measured from on a sloping site?
From the surface of the ground immediately adjacent to the building, and where that surface is not uniform, from the level of its highest part. The limit then applies across the whole building measured from that datum, which is why a level roof over falling ground breaches the limit at the downhill corner.
What is the outbuilding height limit near a boundary?
Under Class E of Part 1, overall height falls to 2.5 metres anywhere within two metres of the boundary of the curtilage. Measure the two metres to the legal boundary on the title plan rather than to the fence, because a fence set inside the boundary moves the line and the limit with it.
Can I excavate to keep an outbuilding within the height limit?
In principle yes, because the adjacent ground becomes the excavated surface, but it has to be real excavation rather than a trench around the walls, and it brings drainage and retaining issues. Authorities and inspectors have reached different conclusions on variants of this, so a Lawful Development Certificate is worth the fee.
Does building up the ground increase my permitted height?
It is unlikely to help. Regrading can itself be development, and permitted development rights for hard surfaces do not cover wholesale reprofiling. A platform more than 0.3 metres high is treated as a raised platform under the Order, and a building on it has its height measured in a way that captures the platform.
How do eaves height and overall height differ?
Overall height is to the highest point of the building, excluding chimneys, flues and antennas where the Order allows. Eaves height is to the base of the eaves where the roof plane meets the wall, assessed for each part of the building. On a slope they must be checked elevation by elevation, not once for the whole building.
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.