Measuring height under Part 1 from adjacent ground
One datum governs the height limits in Part 1: the highest adjacent ground. Here is how it works on slopes, what reprofiling does, and what to show on the section.
Measuring extension height for permitted development starts from one rule that resolves most disputes: height is taken from the highest part of the ground next to the building. Not from the patio, not from the lowest corner, not from the finished floor level. The government's technical guidance for householder permitted development states it plainly, and it applies across Part 1 of Schedule 2 to the GPDO 2015, so the same datum governs the 4 metre limit on a single storey rear extension, the 2.5 metre limit on an outbuilding near a boundary and the eaves limits.
On a level site the rule changes nothing. On a sloping one it decides whether the scheme is lawful.
Where the measurement is taken from
The datum is the ground adjacent to the building, at its highest point. Adjacent means immediately next to the relevant part of the structure rather than anywhere in the garden, so a mound at the far end of the plot is irrelevant.
Two consequences follow, and they run in the applicant's favour.
Where the ground falls away from the house, the highest adjacent ground is near the house, and the measured height of the extension is less than a tape from the bottom of the slope would suggest. Where the ground rises away from the house, the highest adjacent ground is further out, and the measured height is smaller again.
The rule is generous, and it is only useful if the drawings apply it. An officer with a section that shows no ground line cannot give you the benefit of a datum you have not identified.
Why a sloping site changes the answer
Take a rear extension on a garden falling 500 millimetres over its depth. The roof is level. Measured from the top of the slope, next to the house, the extension is 3.6 metres high. Measured from the bottom, 4.1 metres.
Only the first figure matters for Class A, where the single storey rear extension limit is 4 metres. The extension is compliant. But an officer reading a drawing with a single dimension and no levels has every reason to ask which end it was taken from, and the application stalls while you answer.
Cross-slopes are harder. Where the ground falls across the rear elevation rather than away from it, the highest adjacent ground may be at one corner of the extension and the tallest part of the structure at the other. Dimension from the corner that gives the governing datum and mark it.
Steps and retaining walls introduce another question. Where a terrace has been cut into a slope, the ground next to the building is the ground as it exists, including that terrace, so a sunken patio next to the wall lowers the datum rather than raising it.
Eaves height within two metres of a boundary
The eaves limits use the same datum. Class A removes permitted development where the eaves of the enlarged part exceed the eaves of the existing house, and separately where any part of the extension is within 2 metres of the boundary of the curtilage and its eaves exceed 3 metres.
Eaves height is measured to the point where the external wall would meet the upper surface of the roof if the wall continued upwards. So the fascia and gutter hanging below that line do not count against you, and a deep fascia does not tip a compliant extension over.
Because the measurement starts at the highest adjacent ground, a scheme on a falling garden can carry a higher visible eaves on the low side while remaining inside the 3 metre limit. That is lawful and it is also the kind of arrangement a neighbour objects to, so expect the levels to be checked.
Ridge height against overall height
Part 1 does not talk about ridge height. It talks about the height of the enlarged part and the height of the eaves, which means the highest point of the structure and the eaves line respectively.
Three limits are worth holding in mind together.
| Limit | Applies to | Figure |
|---|---|---|
| Height of the enlarged part | Any extension under Class A | Not exceeding the height of the highest part of the roof of the existing house |
| Single storey rear extension | Overall height | 4 metres |
| Side extension | Overall height | 4 metres |
| Eaves within 2 metres of a boundary | Eaves only | 3 metres |
| Outbuilding under Class E | Overall height | 4 metres dual pitched, 3 metres otherwise, 2.5 metres within 2 metres of a boundary |
The Class E figures catch people who have built a garden room to 2.9 metres a metre from the fence. That is a breach by 400 millimetres and there is no tolerance in the wording.
Roof-mounted equipment is measured differently again. A solar panel's 200 millimetre projection limit is measured from the roof plane rather than from the ground, so the two rules operate independently.
Ground reprofiling and its effect
Raising the ground next to a wall does not raise your allowance. The datum is the ground as it was before the development, so imported material against a new plinth will be disregarded, and an officer who sees fresh fill will work from the earlier level.
Reprofiling can also be development in its own right. Substantial regrading is an engineering operation under section 55 of the Town and Country Planning Act 1990 and needs permission unless it falls within a permitted development class of its own. Cutting and filling to create a level garden around a new extension is the common example, and it is often carried out without anyone considering it.
Lowering the ground is the mirror image. Excavating next to a wall after the extension is built lowers the surrounding ground but not the datum that applied at the time, so it does not retrospectively make a compliant extension unlawful. It may, though, trigger a party wall notice and it will affect the neighbour's foundations.
Showing the measurement on a section drawing
This is where applications are won or lost, and the fix takes ten minutes.
- Take spot levels at each corner of the proposed extension on the existing ground, before any work.
- Tie them to an assumed site datum, expressed as a number rather than as a description.
- Draw a section through the extension showing the existing ground line, the proposed ground line where it differs, and the datum.
- Dimension the overall height and the eaves height from the highest adjacent ground, and annotate which level that is.
- Repeat on any elevation where the ground line varies.
Add a short note on the drawing recording the rule you have applied, in one sentence. Officers are not looking for an argument. They are looking for a basis on which to agree.
Where a scheme needs a planning application because the limits cannot be met, a planning statement setting out the levels and the design response does the same job in prose.
What to do when the ground level is disputed
Illustrative example: a garden room built to 2.4 metres at the highest adjacent ground, with the ground falling to give 2.8 metres at the low corner, a metre from the boundary. A neighbour complains. The enforcement officer measures 2.8 metres and opens a case.
The evidence that resolves it is contemporaneous: a pre-construction topographical survey, dated photographs with a levelling staff, or a set of spot levels signed by whoever took them. Without that, the argument comes down to recollection.
Three practical steps where ground level is likely to be contentious.
Commission a short topographical survey before starting, which for a small garden costs a fraction of an enforcement dispute. Photograph the existing ground against a fixed reference such as a door threshold or an air brick course. And where the position is close to the limit, apply for a lawful development certificate under section 192 of the Town and Country Planning Act 1990 before building, which converts a judgement into a document.
Height limits in other parts of Schedule 2 use their own datums and wording, and agricultural classes are a case in point: Part 6 Class B on smaller agricultural units sets height limits that operate differently from Part 1.
Enter your postcode on PlanScope to see which documents an application needs where permitted development limits cannot be met.
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.
Frequently asked questions
Where is permitted development height measured from?
From the highest part of the ground next to the building, according to the government's technical guidance on householder permitted development. Adjacent means immediately beside the relevant part of the structure, so a high point elsewhere in the garden is irrelevant. The same datum governs overall height, eaves height and outbuilding limits under Part 1.
Does a sloping garden help or hinder?
It usually helps. Because the datum is the highest adjacent ground, a garden that falls away from the house gives a measured height lower than a tape from the bottom of the slope would show. The benefit only applies if the drawings identify the datum, so put spot levels at each corner of the extension on the section.
Can I raise the ground level to gain height?
No. The datum is the ground as it existed before the development, so imported fill against a new wall is disregarded. Substantial regrading is also an engineering operation under section 55 of the Town and Country Planning Act 1990 and may need permission in its own right, which is a point frequently overlooked.
What are the height limits for a garden outbuilding?
Under Class E of Part 1, 4 metres for a dual pitched roof, 3 metres otherwise, and 2.5 metres for any part within 2 metres of the boundary of the curtilage. The 2.5 metre figure has no tolerance, so a garden room at 2.9 metres a metre from the fence is a breach even if nobody notices for years.
What evidence settles a dispute about the original ground level?
Contemporaneous records. A pre-construction topographical survey, dated photographs taken against a fixed reference such as a door threshold or an air brick course, or signed spot levels. Without those the argument reduces to recollection. Where the scheme sits close to a limit, a lawful development certificate before building is the safer course.
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.