Measuring height on a sloping plot for permitted development
On a sloping plot, permitted development height is measured from the highest part of the ground next to the building, not from the new floor level. Here is how to find and prove that datum.
Measuring height on a sloping site for permitted development starts from one rule, and it is not the one most people assume. Article 1(3) of the General Permitted Development Order 2015 says that where the ground is not uniform, height is measured from the level of the highest part of the surface of the ground adjacent to the building. Not the average. Not the lowest point. Not the finished floor level. The highest adjacent ground.
On a plot that falls away from the house, that rule works against you. A rear extension that measures 3.9 metres from the patio it stands on can be over the four metre limit when measured from the higher ground at the side.
Why the Order measures from the highest part of adjacent ground
The drafting exists to stop the limits being defeated by a choice of measuring point. If height could be taken from the lowest ground, a house on a bank could put up something far taller than the Order intended, simply by measuring from the bottom of the slope.
The consequence is a single reference level for the whole of the relevant part of the building. You do not measure each elevation from the ground in front of it. You find the highest point of the ground immediately adjacent and measure everything from there.
"Immediately adjacent" is doing work in that sentence. It means the ground next to the building in question, not the highest point anywhere in the garden. A bank at the far end of a long plot is irrelevant. A raised terrace two metres from the extension wall is not.
Finding the reference point on a falling plot
Do this with a level and a tape rather than by eye, because a 300 millimetre fall across a 6 metre width is invisible in a photograph and decisive in a calculation.
- Mark the footprint of the proposed extension or outbuilding on the ground.
- Take spot levels on the existing ground surface at each corner, and at any point where the surface changes noticeably, working around the whole perimeter.
- Include levels on any adjoining surface within a metre or so of the footprint: a path, a drive, a patio, a bank.
- Identify the highest of those readings. That is your datum.
- Measure the proposed overall height and eaves height from that datum, not from where the new slab will sit.
- Record all of it on a drawing with the values, and keep the survey.
Where the fall is substantial, ask the designer for a section along the direction of the slope rather than only across it. Most drawings show the section that flatters the scheme.
Eaves height against overall height
Two separate limits apply in Part 1, and both are measured from the same datum.
Overall height is the top of the roof. For a single storey rear extension the cap is four metres, and there is a separate requirement that the enlarged part must not exceed the height of the highest part of the roof of the existing house.
Eaves height has its own restriction where the extension sits within two metres of a boundary of the curtilage, and the eaves limit in that situation is three metres. This is the limit that catches sloping plots, because it applies to a narrow strip along the boundary and the boundary is often on the high side.
| Element | Where it is measured from | Common trap |
|---|---|---|
| Overall height of single storey rear extension | Highest adjacent ground | Measuring from the new patio level |
| Eaves height within two metres of a boundary | Highest adjacent ground | Forgetting the two metre strip applies at all |
| Height of enlarged part against existing roof | The existing building | Comparing to a later extension rather than the original |
| Outbuilding height | Highest adjacent ground | Dual pitched and other roofs have different caps |
Read the class that applies to your works. An outbuilding under Class E has different height caps from an extension under Class A, and the caps change again within two metres of a boundary.
Retaining walls and made up levels
The temptation on a falling plot is to build up the ground so the datum rises. It does not work, and it can create a second problem.
The accepted approach is to measure from the ground as it exists before the works, so raising levels to gain height is not a route to compliance. Councils and inspectors treat artificially made up ground with scepticism, and an enforcement officer looking at a scheme built on imported fill will ask what the levels were before.
Digging out is a different matter. Excavating to lower the ground next to the building does reduce the adjacent ground level, but the excavation itself may be an engineering operation requiring permission, and the retaining structure needed to hold the slope back is a wall. Walls, fences and other means of enclosure have their own permitted development limits, generally two metres in height and lower where adjacent to a highway, and a retaining wall two and a half metres high in a back garden is development in its own right.
There is also the practical point that a retaining wall against a boundary engages your neighbour, and potentially the Party Wall etc. Act 1996 where excavation is close to their structure.
Showing the datum on a section drawing
A section is the only drawing that proves compliance on a sloping plot, and it needs three things: the existing ground profile drawn as a line, the datum marked with its value, and the proposed heights dimensioned from that datum.
Tie the levels to Ordnance Datum where the surveyor has done so, or to a clearly described site benchmark such as the threshold of the back door, described in words on the drawing. A drawing with levels marked "0.00" and no explanation of what zero is proves nothing.
Add a note stating the rule you have applied: height measured from the highest part of the ground adjacent to the building in accordance with Article 1(3). Officers read that and know you have understood the question, which changes the tone of the correspondence.
Where the slope pushes the design over the limit
You have four options and they are worth considering in this order.
Lower the roof. A flat or shallow mono-pitch roof on the high side, stepping up over the lower side, frequently brings a scheme inside the cap with no loss of usable space.
Move the building. Shifting the footprint a metre away from the boundary removes the three metre eaves restriction entirely, which is often more valuable than the metre of garden it costs.
Drop the floor. A step down into the extension gains headroom without gaining height, at the cost of level access.
Or apply for planning permission. A householder application is not a failure, and on a sloping site the design freedom is usually worth more than the fee and the eight weeks. A permitted development scheme that could lawfully be built also carries weight as a fall-back position in that application, as covered in the permitted development fall-back as a material consideration.
Disputes with a council over the baseline
Disagreements about the datum are common and they are factual, which makes them resolvable.
Where the council says your scheme exceeds the limit, ask which point it has taken as the highest adjacent ground and on what evidence. Then produce the survey. If the difference is real, redesign. If it is a misreading of the plan, a marked-up section usually settles it.
Where the stakes justify it, a certificate of lawfulness of proposed use or development under section 192 of the Town and Country Planning Act 1990 converts the argument into a decision you can rely on and sell with the house. Where the curtilage itself is in question, because land has been added to the garden, the analysis in measuring curtilage and the enlarged garden comes first, since you cannot identify the adjacent ground until you know where the curtilage boundary runs.
Recording levels before groundworks start
Once the digger has been on site for a day, the original ground surface is gone and so is your evidence.
Commission a levelled survey before anything is stripped, keep the surveyor's file, and take photographs with a levelling staff or a marked pole in frame at each corner. Date them. If a question arises three years later at sale, that folder is the difference between a certificate application and an indemnity policy.
Illustrative example: a 1930s semi with a garden falling 450 millimetres from the boundary to the rear of the house. The proposed single storey extension reads as 3.85 metres from the new slab but 4.15 metres from the highest adjacent ground at the side boundary. Moving the roof to a mono-pitch falling away from the boundary brings it to 3.9 metres from the datum, and pulling the flank wall 2.1 metres off the boundary removes the eaves restriction. Details are illustrative.
Where a householder application turns out to be the better route, PlanScope drafts the Planning Statement from your site details, flagging missing facts rather than inventing them. Enter your postcode on PlanScope to see which documents the works are likely to need.
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. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
Where exactly is height measured from on a sloping site?
Article 1(3) of the General Permitted Development Order 2015 says that where ground level is not uniform, height is measured from the level of the highest part of the surface of the ground immediately adjacent to the building. That gives one datum for the whole of the relevant part, rather than measuring each elevation from the ground in front of it.
Can I build the ground up to gain height?
No. The accepted approach is to measure from the ground as it existed before the works, so raising levels with imported fill is not a route to compliance and councils treat made up ground with scepticism. Excavating to lower the adjacent ground does change the datum, but the excavation and any retaining wall may need permission.
What is the eaves height limit near a boundary?
Where an extension under Part 1 Class A sits within two metres of a boundary of the curtilage, the eaves height must not exceed three metres, measured from the same highest adjacent ground datum. On a plot that rises towards the boundary this is usually the limit that fails first, ahead of the overall height cap.
How do I prove the datum to the council?
With a section drawing showing the existing ground profile as a line, the datum marked with its value, and the proposed heights dimensioned from it. Tie levels to Ordnance Datum or to a clearly described site benchmark, and add a note stating that height has been measured in accordance with Article 1(3).
What if the slope puts the design over the limit?
Consider a shallower or mono-pitch roof falling away from the high side, moving the footprint beyond two metres from the boundary to remove the eaves restriction, dropping the internal floor level, or applying for planning permission. On a sloping site the design freedom a householder application buys is often worth the fee and the eight weeks.
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.