Floor levels and thresholds where a garden slopes away

Where a rear garden falls away from the house, the extension floor level on a sloping garden is the first thing to fix and the last thing most people think about. Everything follows from it: whether you get a level threshold, how much retaining structure you need, where the surface water goes, and whether the extension stays within permitted development height limits at all. Decide it before the plan layout, because the plan can be adjusted and the ground cannot.

The good news is that the permitted development rules measure height in a way that helps you on a falling site. The less good news is that the detailing at the threshold is where the cost sits.

Why the garden level decides the design

An extension built out from the back of a house on a falling garden has three possible relationships with the ground. The floor can continue the internal level of the house, which means the new floor sits above the external ground and you need a step down to the garden. The floor can drop to meet the garden, which means a step down inside the house. Or the ground can be raised to meet the floor, which means retaining structure and a raised platform.

Each has a consequence that an officer or a neighbour will notice. A floor at house level over a falling garden produces a visible plinth, and on a steep fall that plinth becomes a wall. A dropped internal floor is comfortable in use and awkward where it meets existing door openings and radiator pipework. Raising the garden alters levels on the boundary, which is the version most likely to generate an objection.

Measure the fall first. Spot levels at the corners of the proposed footprint and at the boundary, referenced to a datum, cost very little and change the design conversation completely.

Level access against a step down

A level threshold between an extension and a garden is achievable on a falling site, and on a modest fall it is usually the right answer. What it requires is a drainage channel immediately outside the opening and a threshold detail designed for the exposure.

The constraint is damp. Approved Document C's recommendation of 150mm between the finished external ground level and the damp-proof course exists for a reason, and a level threshold reduces that margin to nothing. The accepted detail is a slot drain or grated channel across the full opening width, connected to a surface water outlet, with a proprietary low-upstand threshold section.

Part M of the Building Regulations sets accessibility requirements for new dwellings and does not generally apply to an extension of an existing house, so a level threshold on a householder extension is a design decision rather than a requirement. Confirm the position with your building control body, since the treatment of material alterations varies with what else the project involves.

Where the fall is greater than about 300mm across the footprint, a single step down outside the opening usually beats either a long ramp or a deep plinth. One generous step with a landing reads better than three shallow ones.

Plinth walls, retaining and the boundary

Once the floor sits above the garden, the perimeter becomes a retaining problem. A plinth of a couple of courses is a detail. A plinth of a metre is a retaining wall with surcharge from the extension foundation behind it, and it needs designing.

Three points cause arguments:

• Ownership of the boundary. A retaining structure that bears on or against a neighbour's land is a different proposition from one wholly within your own. Establish the boundary position from the title plan and on site before designing it.
• Excavation close to a neighbouring structure. The Party Wall etc. Act 1996 notice requirements bite on excavation within three metres of an adjoining owner's structure below its foundation level, and within six metres on a defined slope. Those are civil obligations running alongside planning rather than part of it.
• Changes in level at the boundary. Raising ground against a neighbour's fence or wall transfers load to it and changes drainage. Expect to be asked about both.

A raised terrace deserves separate thought. Class A of the General Permitted Development Order 2015 does not permit a verandah, balcony or raised platform, and a raised platform is defined as one more than 300mm above the ground. So a deck at floor level over a garden that falls 600mm needs planning permission even where the extension itself does not, and the overlooking questions that come with a raised terrace or Juliet balcony then arrive with it.

Drainage falls away from the threshold

Water runs downhill towards the house on a falling garden only if the ground immediately outside has been built up wrongly. Get the external falls right and the threshold detail has much less work to do.

The sequence that works is a fall of around 1 in 60 away from the building across any paved area, a channel drain at the threshold where the threshold is level, and a surface water destination that is not the foul system. Soakaways should sit at least five metres from building foundations under Approved Document H, which on a small garden with a falling slope can be the hardest requirement to satisfy. Rainwater harvesting or a simple green roof with attenuation on the extension itself reduces the volume you have to dispose of.

If the garden drains towards a neighbour at the bottom of the slope, do not increase what arrives there. That is a civil matter rather than a planning one, and it is also the complaint that sours a relationship for years.

Permitted development height on a falling site

This is where the falling garden helps. The householder technical guidance for permitted development explains that where the ground is not uniform, height is measured from the highest part of the surface of the ground next to the building. On a garden that falls away, the highest adjacent ground is at the house, so the 4 metre limit on a single storey rear extension and the 3 metre eaves limit within 2 metres of a boundary are measured from the higher level.

The practical effect is that an extension can be physically taller above the lower end of the garden while remaining within the limits. Do not stretch this. The measurement is from ground next to the building, not from the highest point anywhere in the garden, and an officer assessing a certificate application will take the levels from your own section drawing.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it, and remember that a Lawful Development Certificate under section 192 of the Town and Country Planning Act 1990 is the only formal way to confirm that works would be lawful without permission.

Showing levels on the section drawing

A section is the drawing that proves the design, and most refused or invalidated householder applications on sloping sites have a weak one.

Put the following on it:

1. Existing ground profile across the full depth of the garden, with spot levels to a stated datum.
2. Proposed finished floor level, with the relationship to the existing internal floor.
3. Threshold detail, including the channel drain where used, and the damp-proof course level.
4. Proposed external levels including any plinth, retaining structure or terrace.
5. Eaves and ridge heights, dimensioned from the ground level used for the permitted development measurement.
6. The boundary position and the height of any existing boundary structure.

Label the datum. A section with levels given as plus or minus figures against no stated origin is impossible to check, and that alone can prompt a request for further information.

An illustrative example

A 1930s semi has a rear garden falling 750mm over the first 4 metres. The extension floor continues the internal level, producing a 600mm plinth at the rear corner. The design uses a 150mm step down to a paved terrace at 450mm above the lower garden, which stays under the 300mm raised platform threshold at its own perimeter by setting the terrace into the slope. A channel drain runs across the 3.6 metre opening to a soakaway 5.5 metres from the foundation. This example is illustrative and not a real project.

A short Planning Statement is the right place to explain the level strategy in words, particularly where a plinth or retaining wall sits near a boundary.

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