Basement Extension London

Below ground is the only direction left on a lot of London plots. The garden is already short, the loft is already converted, and the neighbours are attached on both sides. A basement extension adds floor area without touching the footprint, the roofline or the garden, which is why it survives planning in places where nothing above ground would.

It is also the most technically demanding thing you can do to a house, and the gap between doing it properly and doing it cheaply shows up as water. Everything on this page is about that gap.

Cellar conversion or a new basement? They are not the same project

These two get quoted as though they were one thing and they are not, so the first question at survey is always which one you actually have.

A cellar conversion takes an existing below ground space, most often the Victorian coal cellar or vault under the front rooms, and makes it habitable. There is no excavation of a new void, and often only a modest dig to gain head height. The work is waterproofing, drainage, ventilation, light, insulation and finishes. In London that runs roughly £1,200 to £2,250 per square metre.

A basement extension is an excavation. The ground under or beside the house is dug out, the existing walls are underpinned in sequence to take the house down to the new level, a new structural box is formed and waterproofed, and the spoil is taken out through the house or the garden. In London that runs roughly £3,000 to £5,000 per square metre, and a whole project on a terrace commonly totals £100,000 to £250,000 or more.

If you have an existing cellar with reasonable head height, say the answer honestly to yourself before you spend money on a full basement scheme, because the cheaper project may well give you the room you wanted. Our London basement conversion cost guide sets the two side by side line by line.

Underpinning, sequencing and the structural design

Underpinning is what makes a new basement possible. The existing foundations sit at their original level, so the ground beneath them is removed and replaced with new mass concrete or reinforced sections taken down to the new basement floor, transferring the load of the house onto them.

The critical thing is that it is done in short sequenced bays rather than in one run. Typically that means bays of around a metre, excavated and cast in a hit and miss pattern with only a small proportion of the wall unsupported at any one time, in an order set out by the structural engineer. That sequence is the design. Departing from it is how houses crack, and it is not a decision for the site.

To be entirely clear about who does what: the structural engineer designs the underpinning, the sequence, the retaining structure and the temporary works. GS Renovation is the contractor. We build to that design, we work to the specified sequence, and building control and the engineer inspect as the work goes. We do not produce structural calculations and would not build to a basement design that did not come from a qualified engineer.

• A ground investigation and, where required, a basement impact assessment, because many London boroughs demand one before consent
• Structural engineer's design covering underpinning, the retaining walls, the sequence and the temporary propping
• Excavation and spoil removal, which on a terrace with no side access means every barrow load comes out through the house or over a conveyor
• Underpinning cast in sequenced bays, inspected before each pour
• A new slab, drainage and a sump chamber with pumps where the drainage cannot run to the sewer by gravity
• The waterproofing system, designed to BS 8102 and installed before anything is boarded or plastered
• Light and escape: a light well, a rear lightwell or a widened stair, both for daylight and for the fire escape route
• Ventilation, insulation, heating and the finishes that make it a room rather than a void

Waterproofing designed to BS 8102

Water in the ground finds any route it can, and below ground you cannot rely on one layer to stop it. BS 8102, the British Standard for the protection of below ground structures against water ingress, is the framework a competent basement is designed to. It sets out three types of protection: Type A, a barrier applied to the structure; Type B, protection built into the structure itself; and Type C, a drained cavity system that collects any water reaching the inside face and takes it to a sump.

Two points matter more than the labels. The first is that habitable space normally warrants combined protection rather than a single system, because a single system with a single defect is a leak. The second is that any drained system has to be maintainable: accessible drainage channels, an accessible sump, dual pumps and a way of testing them. A basement whose pump chamber has been tiled over is a basement waiting to flood.

The waterproofing design should be produced by a specialist, and on our projects it is. We install to that design and we do not improvise it on site. If you take one thing from this page, take that the waterproofing is a designed system with a named designer, not a bucket of tanking slurry.

Party wall, planning and your neighbours

The Party Wall etc. Act 1996 is close to unavoidable on a basement. Excavating within 3 metres of a neighbouring building and to a greater depth than their foundations requires notice, and the 6 metre rule brings in deeper excavations further away. Underpinning a party wall requires notice in its own right. On a terrace you are usually serving on both sides, and the neighbours will almost always appoint a surveyor, so a party wall award with a schedule of condition is the normal outcome.

The duty to serve those notices sits with you as the building owner. We are not party wall surveyors and we do not act as one. What we do is tell you at quotation stage precisely which notices are engaged, how long the statutory response periods run and what the award is likely to require of the works, so that the programme is built around reality. Our Party Wall Act guide explains the notice periods.

On planning, a basement wholly beneath the existing footprint with no external alteration can sometimes be permitted development, but that is the exception rather than the rule in London. Light wells, external stairs, altered front elevations and any extension beyond the footprint all need consent, and a large number of London boroughs have specific basement policies limiting depth, footprint, the proportion of garden that may be built under and requiring a basement impact assessment on drainage, groundwater and structural stability. Assume you need permission and be pleased if you do not. In a conservation area or on a listed building, assume it and add listed building consent.

What a basement costs in London

Scope	What it covers	Typical London cost
Cellar conversion	Existing below ground space made habitable: waterproofing, drainage, ventilation, insulation and finishes	£1,200 to £2,250 per m²
New basement excavation	Excavation, underpinning, structural box, waterproofing and fit out	£3,000 to £5,000 per m²
Whole basement project, London terrace	Typical completed project including underpinning, waterproofing and finishes	£100,000 to £250,000+
Underpinning	The single largest line in most basement budgets	£1,500 to £2,600 per m²
Party wall surveyors	Per adjoining owner, with a single agreed surveyor, roughly double if each side appoints their own	£1,800 to £2,700
Professional fees	Structural engineer, waterproofing designer, architect, basement impact assessment, building control	10 to 15 per cent of build cost
Contingency	Ground conditions, water, and what is found under a Victorian house	10 to 15 per cent

Indicative London ranges, 2026. Trade and project costs from MyBuilder and Checkatrade; per square metre build rates from Architecture for London and Resi.

The uncomfortable arithmetic is worth stating. Below ground space is normally valued below above ground space because it has less daylight, so a basement that costs £5,000 per square metre to build in a street selling at £8,000 per square metre is a thinner proposition than it first appears. A basement is usually the right answer when there is genuinely nowhere else to go and you intend to stay, and the wrong answer when it is being done purely as an investment. We will say so at survey.

Living with the work

A new basement is the one project where we would usually advise moving out for the excavation and underpinning phase. The house is on temporary support, the noise is constant, and the spoil route often runs through the middle of the ground floor. A cellar conversion is a different matter and most clients stay put throughout.

Either way, damp is the thing to understand before you start rather than after. Our guide to damp causes and fixes explains why a below ground room behaves differently from the rest of the house, and why ventilation matters as much as waterproofing.