Downhill From the Door: External Levels, Thresholds and Drainage Around a Three Counties Extension

An extension is usually judged from the inside first: the new kitchen, the wider opening, the view into the garden. The ground outside the doors is treated as a finishing trade, something to be decided once the scaffolding has gone. In practice, external levels are one of the few details that can quietly undermine an otherwise competent build. A patio laid a course too high can bridge a damp-proof course. A path falling the wrong way can send rainwater towards a wall. A threshold set without thought for drainage can leave a door that closes perfectly but still lets water track indoors.

In the Three Counties, where older properties, sloping plots and heavy soils are common, these are not theoretical risks. They deserve to be designed before the landscaper arrives, not corrected afterwards.

Begin at the finished floor, not the paving

The starting point is the finished floor level of the extension and the threshold detail at each external door. Everything outside should be set out from those two references. The common mistake is to lay a patio or path first and then discover that it sits awkwardly against the door, leaving a step that is too high, a fall that is too shallow or a damp-proof course that has been covered.

Before any external works begin, confirm the relationship between the internal floor, the door threshold and the proposed external paving. On a typical extension, the damp-proof course should remain at least 150mm above finished external ground level, although the exact figure should be confirmed with the designer and building control for the particular construction. That clearance is not decorative. It is part of the wall's defence against rising and penetrating damp.

Where the existing house has a low damp-proof course, raising the garden to meet a new door can compromise the older building as well as the extension. A change in level may be better handled with steps, a drainage channel or a redesigned threshold rather than by bringing the ground up to the door.

The damp-proof course is a line, not a suggestion

Damp-proof courses, cavity trays and air bricks are easily buried by well-intentioned landscaping. Gravel boards, raised beds and block paving can all reduce the clearance a wall needs to shed water and ventilate. The same applies to render and cladding that continue below the intended line.

A practical check is to walk the external walls before the final surfaces are laid and mark the damp-proof course, air bricks and any weep holes. The finished ground level should sit below those marks, with a visible margin. Where a patio must meet a wall, a narrow gravel strip or drainage channel can help, but it does not replace the required clearance.

This matters particularly in older Three Counties properties where brickwork, stone and render may already be damp-sensitive. A raised patio that looks neat against the wall can create a slow, persistent problem that only appears after a wet winter.

Thresholds: the detail that decides whether the door works

A threshold has two jobs: it must allow the door to operate and it must manage water. These aims can pull in different directions. A flush threshold gives a clean route from kitchen to garden, but it leaves very little margin for error if the paving outside is not falling away correctly.

Where level access is required, Approved Document M sets tight limits on threshold heights, and these should be resolved at design stage rather than by raising the external ground. A drainage channel immediately outside the door, set below the threshold, can help manage surface water without compromising the door. The channel must itself have a fall and a clear outlet; a channel that merely collects water and holds it is not a solution.

For sliding doors, the track and drainage detail deserve particular attention. Water should not rely on sealant alone. The paving or deck outside should fall away from the track, and any drainage outlet should be accessible for clearing. Door thresholds should be checked after the external surfaces are laid, not just when the frame is installed.

Falls, gullies and the route water takes

External paving around a house should generally fall away from the building. For patios, a fall of between 1:60 and 1:80 is often used, with the precise figure depending on the surface and the designer's specification. Paths and other surfaces may need a different fall. What matters is that the direction of fall is deliberate and continuous, not interrupted by a raised edging or a section laid flat.

Water should have a clear route to a gully, channel or soakaway. On a sloping plot, it may also need to be intercepted before it reaches the extension. In parts of the Three Counties with clay soils, soakaways may not be suitable, and infiltration should be checked rather than assumed. Downpipes should connect to a drainage system or discharge to a suitable surface, not simply splash onto the wall or patio.

Gullies and channels need to be installed before paving is laid, with falls set to their outlets. If a drainage run is covered before it has been inspected, correcting a problem later becomes expensive and disruptive.

A 2026 sequencing checklist for external works

The following sequence is a practical framework for coordinating landscaping, drainage, access and building control on a Three Counties extension.

1. Confirm the finished floor level, damp-proof course level and threshold details before ordering paving or decking.
2. Mark buried services, drainage runs and air bricks, and agree protection measures with the contractor.
3. Install drainage, gullies and channels before laying sub-base or paving.
4. Construct steps, retaining walls and raised beds with suitable drainage behind them.
5. Lay sub-base and paving to the agreed falls, keeping the damp-proof course and air bricks clear.
6. Check door operation and threshold drainage before the final surfaces are cleaned down.
7. Arrange any building-control inspection of drainage or external levels before work is covered.
8. Record as-built levels, drainage routes and product details for the handover file.

What building control will want to see

External landscaping is not usually the main subject of a building-control inspection, but the levels around the building can affect compliance with damp-proofing and drainage requirements. The inspector may ask how the damp-proof course is protected, how surface water is dealt with and whether the drainage has been tested or inspected before being buried. Keeping a simple record of levels and drainage runs will make the final certificate less awkward to obtain.

The last check is the one that matters

Once the patio is laid and the paths are finished, test the practical details. Open and close every external door. Pour water onto the paving near the threshold and watch where it goes. Check that gullies are clear and that air bricks remain visible. Look for any low spots where water might sit after rain.

These are small observations, but they are the ones that reveal whether the ground outside the extension has been set to work with the building rather than against it. In a Three Counties garden, with its slopes and wet winters, the fall of the land is not a finishing touch. It is part of the construction.