Thresholds and level access on a sloping plot

A flush threshold between an extension and a garden that falls away from the house is a three way argument between access, drainage and damp, and the drawing that resolves it is a 1:5 section rather than a note on the plan. The access case wants no step. The damp proof course wants to sit clear of the finished external level. The drainage wants somewhere for water to go before it reaches the door. On a sloping plot you can satisfy all three, but only by designing the threshold, the channel and the external levels as one detail.

Planning officers rarely refuse an extension over a threshold detail. They do refuse schemes where the garden has been regraded, a retaining wall has appeared on the boundary and a raised terrace now looks into next door's kitchen.

Why a flush threshold is harder than it looks

Traditional practice put a step at the door for a reason. A damp proof course is normally set clear of the external ground so splashing and saturated paving cannot bridge into the wall, and the simplest way to achieve that is to raise the internal floor above the paving.

Remove the step and you have to replace that protection with something else. The usual answer is a drainage channel immediately outside the door, a positive fall away from the building, and a cavity tray or proprietary threshold unit that keeps the barrier continuous where the floor and wall meet.

Three failures recur. A channel with nowhere to discharge, which fills with silt and then floods the door. A threshold set flush with paving that was laid level rather than falling away, so water stands against the frame. And a detail drawn correctly on section but built by a groundworker who was not shown that drawing.

The drainage channel detail that works

The channel has to take the water, hold its fall and be cleanable by whoever lives there.

• A linear channel with a removable grating, set the full width of the opening and returning at least 150mm past each jamb, so water running along the wall is intercepted.
• A fall within the channel towards an outlet, with the outlet connected to the surface water system rather than discharging against the foundation.
• Paving laid to fall away from the building, commonly around 1 in 60 to 1 in 80 for a hard surface, so the channel is a backstop rather than the primary route.
• A gravel margin or slot drain where a grating would read poorly, provided it still has a positive outlet.
• A soakaway or permeable area sized and sited away from the foundation, where the surface water cannot be connected to an existing drain.

Where the extension increases the hard surface area, check what the increase does to surface water runoff and where it goes. Approved Document H covers the drainage requirements, and discharging a new channel into a combined sewer may need the agreement of the sewerage undertaker.

Finished floor level against external ground

Set the internal floor level first, from the existing house, and then work the external levels to it. The alternative, setting the garden level and working inwards, produces a step inside the house between the old and new floors, which is worse than a step at the door.

On a plot that falls away from the house, the external level at the door is already lower than the internal floor, which is the easy case. The detail becomes a threshold unit, a channel and a short run of paving falling to the lawn or to steps further out.

On a plot that rises away from the house the problem reverses. Water runs towards the building, the ground is above the damp proof course somewhere along the elevation, and you need a cut and a retaining structure with land drainage behind it. That is a structural and drainage design question rather than a detailing one, and the cost is in the retaining wall.

Where the two meet, a cross fall along the rear elevation, the levels have to be set at both ends of the extension and the threshold designed for the worst one.

External steps, ramps and handrails

Once you have the threshold flush, the level change has moved outwards into the garden, and it has to be dealt with somewhere.

Take the exact figures from Approved Document M and the relevant Approved Documents rather than from this table, because they are set out there with the conditions attached and they are revised from time to time.

Part M applies to new dwellings rather than to extensions of existing houses, and work on an existing dwelling must not make the building less compliant than it was. What that means in practice is that level access to an extension is usually a design choice rather than a legal requirement, which is why it is worth raising in the Design and Access Statement rather than leaving it to building control.

Where the garden level has to change

This is the part that attracts planning attention. Raising the ground level to meet a new floor level creates a raised platform, and a raised platform is excluded from the permitted development rights for extensions under Part 1 of the General Permitted Development Order 2015, with 300mm the figure generally used as the limit for a platform.

So a terrace raised 450mm above the natural ground level to give a flush threshold is not permitted development even if the extension behind it would have been. It needs an application, and it will be assessed on overlooking, because standing on a raised terrace at first floor sill height of the neighbour's rear window changes what each party can see.

Three ways through it:

1. Keep the raised area small, immediately outside the door, and step down to the existing garden level within a metre or two.
2. Use the fall to your advantage with a sunken terrace cut into the slope, which gains privacy rather than losing it.
3. Screen the raised area with a boundary treatment of a height that itself falls within permitted limits, and show the sections through the boundary on the drawings.

Large scale regrading, retaining walls and the import or export of soil can amount to engineering operations needing permission in their own right. Where the works run close to a boundary, the eaves and boundary issues in our note on eaves height limits when an extension sits near a boundary apply to the levels too, and on a corner plot the exposure is greater again, as our guide to extending a house on a corner plot sets out.

What the section drawing should show

A case officer and a building control surveyor read different things from the same drawing, and one drawing can satisfy both if it carries the information.

Show a long section through the extension and out into the garden, with existing ground level and proposed ground level drawn as separate lines and labelled with levels to a stated datum. Show the internal finished floor level, the damp proof course, the threshold, the channel and its outlet, and the paving falls as arrows with gradients.

Then add a 1:5 detail of the threshold itself: the frame, the threshold unit, the insulation line, the damp proof membrane and its lap, the cavity tray, the channel and the paving. Annotate the 15mm maximum upstand if you are claiming level access, because that is the figure the surveyor checks.

Put the levels on the block plan as well. Existing and proposed spot levels at each corner of the extension, at the boundary and at the far end of the garden tells the officer more about the scheme than any amount of text in the Planning Statement.

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 without planning permission. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. Enter your postcode on PlanScope to see which documents a sloping site extension is likely to need.