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This is the part homeowners skip and contractors should not. Approved Document H sets the order of priority for rainwater: an adequate soakaway or some other adequate infiltration system, or where that is not reasonably practicable a watercourse, or where that is not reasonably practicable a sewer. Groundwater and surface water are not supposed to be dumped into a foul sewer, and doing it is one of the causes of the sewer surcharging that floods other people's houses.

Where the answer is a soakaway, Approved Document H's infiltration guidance limits where it can go: infiltration devices should not be built within 5m of a building or road or in areas of unstable land, nor in ground where the water table reaches the bottom of the device at any time of the year. On heavy clay in a small garden those two constraints together frequently rule infiltration out, which is when the conversation turns to a ditch or a watercourse.

A ditch brings responsibilities. Environment Agency guidance is that you normally own a stretch of watercourse that runs on or under your land, or on the boundary of your land up to its centre, and a watercourse includes a ditch or a culvert. As an owner you must let water flow naturally, and you must not build anything which could divert water and increase flood risk to other people's property, or you may have to pay damages. Permission is needed to build anything in or around the watercourse, and on a watercourse that is not a main river that consent comes from your lead local flood authority or the internal drainage board under the Land Drainage Act. The guidance also warns that risk management authorities are unlikely to give permission to build a culvert, and that permission is needed for work within 8 metres of a flood defence, or 16 metres of a tidal one.

A competent job starts with levels and an outfall, not with a digger. Someone establishes where the water is coming from, where the ground falls, and where the drain can discharge to with the necessary consent — and if there is no viable outfall, an external land drain is the wrong solution and should be abandoned before anyone digs. A drain that discharges into a hole in a clay garden is a pond with a pipe attached.

The trench is then cut to a consistent fall, the perforated pipe bedded in clean washed stone, and the stone wrapped in a geotextile so that silt from the surrounding soil does not migrate in and choke the voids. Where the drain has to pass close to a wall, the depth is a structural question rather than a drainage one — excavating below the level of a shallow strip footing, which is what most older British houses sit on, can undermine it, and near a neighbour's wall it can engage the Party Wall etc. Act 1996 in England and Wales. Access points are formed so the run can be rodded or jetted later.

Approved Document C has a specific rule that catches extensions and new drives: if an active subsoil drain is cut during excavation and it passes under the building, it should be re-laid in pipes with sealed joints and have access points outside the building, or re-routed around the building, or re-run to another outfall. Cutting a field drain and leaving it discharging into the ground beside a new foundation is a recognised way to create a damp problem where there was none.

For a wet garden, a saturated lawn, water running towards the house or a damp outbuilding, external drainage is the natural fit: it deals with the water before it reaches the building, it is maintainable, and it does not touch the structure. It is also the only approach that helps with surface water arriving from somewhere else on the plot.

For a basement or cellar that is to be used as a room, external drainage alone is rarely the whole answer in Britain, because our below-ground spaces are usually brick, usually old, and usually built with no waterproofing at all. BSI describes BS 8102:2022 as covering three approaches to keeping water out of structures partly or wholly below ground: waterproofing barrier materials applied to the structure, structurally integral watertight construction, and drained cavity construction. In a conversion of an existing cellar, drained cavity construction is normally the practical option, because it manages water that gets in rather than trying to hold back groundwater pressure with a coating applied to a wall of uncertain condition.

The two are not alternatives so much as different scales of the same problem, and a good survey will often recommend both: reduce the water reaching the structure outside, and manage what still gets through inside. What should worry you is a quotation for one without any assessment of the other. LokalMatch exists to get you to the trades, not to choose between them: describe the ground, the building and your postcode once, and drainage and waterproofing contractors working in your area get in touch, paying LokalMatch for the requests they take. LokalMatch does none of the digging, sets no rates, and does not vet, rank or recommend anyone who contacts you, so comparing what each firm proposes to do about the outfall is the part you should not delegate.