Basement conversions in London: what's involved

Going down is often the only way left to add space to a London house: the garden is small, the loft has been converted and there is nowhere else to extend. A basement can add a whole new floor, but it is the most specialist domestic building project there is, and in London it is also one of the most tightly regulated. This guide explains the two kinds of basement project, how they are built and kept dry, and how the rules differ from one borough to the next.

Cellar conversion or new basement?

There are two very different starting points. Many Victorian and Edwardian houses already have a cellar, often low, damp and used for storage. Converting it usually means lowering the floor to gain headroom, underpinning the walls where needed, installing a waterproofing system and then insulating, wiring and finishing the space. It is a substantial job, but the structure is largely already there.

A new basement means excavating beneath a house that has none, or extending an existing cellar under the whole footprint or into the garden. The house is supported while the ground beneath it is removed in carefully sequenced sections, new retaining walls and a new floor slab are cast, and the space is waterproofed before any fit-out begins. It is a much bigger undertaking, closer to civil engineering than to a typical extension.

How a basement is built

Excavation under an existing house is usually done by underpinning: digging out and casting new foundations in short sections, in a set sequence, so the walls above are never left unsupported over a long length. Temporary propping holds the structure while each section is formed. Spoil is removed by hand or by conveyor to a skip or grab lorry, which is why access, parking and any skip or scaffold licences are planned before work starts.

Every stage is designed by a structural engineer and checked by building control, and the order of work is set out in a construction method statement. On a terrace, the party walls are underpinned too, which brings in the Party Wall Act, covered below.

Keeping it dry: waterproofing

Waterproofing is the heart of a basement, and it is designed to the British Standard for protecting below-ground structures from water, BS 8102. The standard describes three forms of protection: a barrier applied to the structure, a structure that is itself watertight, and a drained cavity system, where a membrane fixed inside the walls and floor collects any water that gets through and channels it to a sump, from which a pump removes it.

Most domestic basements use a drained cavity system, often combined with another form of protection for extra security. Because it relies on pumps, a good design includes a backup pump or battery backup and a way of accessing the sump for maintenance. Protection against sewer water backing up into the basement is also required, usually by a pumped device or a non-return valve on the drainage.

Planning: every council has its own basement policy

Basements are one of the few areas where London councils have written detailed policies of their own, largely because of their impact on neighbours during construction. The details differ, but the themes are consistent: a limit on depth, usually one storey; a limit on how far a basement can extend under the garden; soil and drainage above any part under the garden; and supporting documents such as a basement impact assessment and a construction management plan.

In Richmond and Fulham, borough-wide Article 4 directions mean every basement needs planning permission, even one that would otherwise count as permitted development. Richmond's Local Plan resists basements of more than one storey and limits them to half of the garden; Hammersmith & Fulham's policy limits how far a basement can extend under the garden and requires a construction method statement by a qualified engineer. Merton's Local Plan, which applies in areas such as Wimbledon, limits a basement under an existing home to a single storey and requires a basement impact assessment, drainage strategy and construction traffic management plan. Wandsworth's policy, covering areas such as Tooting, allows one storey and limits a basement to half of the rear garden. Each of our area pages summarises the local policy with a link to the council document.

In conservation areas, basements and especially their lightwells must also preserve the character of the street, so front lightwells in particular are closely scrutinised.

Flood risk

Flood risk shapes basement proposals more than any other extension. Parts of South West London lie in the flood zones of the Thames, the Wandle and their tributaries, and several councils restrict basements accordingly. Richmond does not permit basements in the functional floodplain and limits their use in higher-risk parts of Flood Zone 3; Wandsworth does not allow basement bedrooms in its higher flood zones; and Hammersmith & Fulham does not permit new self-contained basement flats in areas at risk of rapid flooding without a safe means of escape.

Surface water matters too. In Colliers Wood, Merton's investigation into the September 2024 floods found some streets sit well below their surroundings. Check your property's flood zone on the Environment Agency's flood map and your council's flood risk pages before designing anything below ground; a flood risk assessment is usually part of the application in the higher-risk zones.

Neighbours and the Party Wall Act

Basements almost always fall under the Party Wall etc. Act 1996, because they involve excavating close to, and usually below, the neighbours' foundations. Notices must be served before work starts, neighbours commonly appoint their own surveyor, and a detailed schedule of condition of the neighbouring property is taken at the outset. Our guide to the Party Wall Act explains the process. Good communication with neighbours throughout, about noise, working hours and deliveries, makes an enormous difference on a basement project.

Light, ventilation and fire safety

A basement room is only as good as its light and air. Lightwells, glazed walkways and roof lights over the garden part of a basement bring daylight down; mechanical ventilation keeps the air fresh and controls humidity. Building Regulations require a safe means of escape from habitable rooms, which in practice shapes where bedrooms can go and whether an escape window into a lightwell is needed, alongside smoke alarms and fire-resisting construction.

How long it takes

Timescales vary more than for any other project. A straightforward cellar conversion may take a few months on site, while a full excavation beneath a house can take the better part of a year, with the approvals, surveys and party wall process adding time beforehand. You should receive a programme specific to your project before work starts. Our basement conversions page explains how we run a basement project from feasibility to handover.