A basement conversion is the only way to add a full extra floor to a London home without changing its footprint or its roofline, and it is also the most technically demanding project a builder can take on. In 2026, London basement conversions range from around £45,000 for fitting out an existing dry cellar to £180,000 to £350,000 or more for a full new excavation beneath an occupied house.

At a glance

  • Existing cellar conversion: £45,000 to £80,000, waterproofing and fitting out a space that already exists
  • Basement lowering with underpinning: £95,000 to £180,000, digging down within the existing footprint to increase head height
  • New basement excavation: £180,000 to £350,000+, digging an entirely new floor beneath the house
  • Typical timeline: 12 to 20 weeks for an existing cellar, 32 to 52 weeks for a full excavation
  • Planning: full planning permission and a party wall agreement are required for almost every basement project in London

The three types of basement project

Existing cellar conversion

Many Georgian and Victorian London houses already have a cellar, often used only for storage. Converting it into habitable space usually means installing a cavity drain waterproofing system compliant with BS 8102, improving ventilation, adding proper lighting and electrics, and building a compliant staircase. Because the excavation already exists and the external appearance of the house does not change, this is the fastest and least disruptive basement project.

Basement lowering

Where an existing cellar has inadequate head height, typically below the 2.2 to 2.4 metre minimum for habitable use, the floor slab can be broken out and lowered. This is structurally more involved than a straight conversion because it changes the bearing conditions of the surrounding walls, and it usually requires underpinning the perimeter foundations in sequenced sections.

New basement excavation

Digging an entirely new basement beneath a house with no existing cellar means underpinning every section of the external walls in a controlled sequence while excavating beneath an occupied building. This is the most complex and most expensive residential building work in London, and it is not a project to price on a rough guide alone. A full structural survey and geotechnical assessment come before any figure is agreed.

What a basement conversion costs in London in 2026

Per square metre, expect £1,800 to £2,800 for an existing cellar conversion, £4,000 to £5,500 for a lowered basement, and £4,800 to £6,500 for a full new excavation. For a typical 40 to 50 square metre basement under a three-bedroom terrace, a full new-dig project commonly lands between £180,000 and £300,000 before fit-out. Adding a lightwell for natural light typically adds £15,000 to £25,000.

Ground conditions drive much of the variation. London’s clay geology is generally workable but demands careful temporary support during excavation, and a high water table or poor access, where materials must be hand-carried rather than machine-loaded, can add significantly to both cost and programme.

Planning permission and party walls

Almost every basement conversion in London needs full planning permission, particularly where it involves excavation beneath the existing footprint or the addition of a lightwell. A party wall agreement with every adjoining owner is standard practice given the proximity of the works to shared foundations, and on a full excavation this can involve several separate surveyors depending on how many neighbours are affected.

Building Control inspects at every stage of underpinning and waterproofing, and a structural engineer’s sign-off is required before the project can be signed off. We coordinate the engineer, the party wall surveyors and Building Control as a single managed process rather than leaving you to chase three separate parties.

Our basement process

We start with a structural survey and a ground investigation to establish soil type, water table and the condition of existing foundations. From there, a structural engineer designs the underpinning sequence, we obtain planning permission and agree party wall matters with neighbours, and only then does excavation begin, one carefully propped section at a time.

Waterproofing is never an afterthought on our basement projects. We design and install a full BS 8102 compliant system, whether that is tanking, a cavity drain membrane, or a combination of both, appropriate to your specific ground conditions.

Why choose Alpha London Builders for your basement conversion

Basement work has no tolerance for shortcuts, which is why we manage the structural engineering, underpinning sequence and waterproofing design in-house rather than handing the technical risk to a subcontractor. Every basement we build is signed off by Building Control before we consider the job finished, giving you the paperwork you will need if you ever come to sell.

Frequently asked questions

Will excavating a basement damage my house or my neighbour’s

Not when the underpinning is designed and sequenced correctly by a structural engineer and carried out in small, propped sections. This is precisely why basement work costs more than surface-level building work, and why we do not cut corners on the engineering.

How long will I need to move out

Most clients remain in the house throughout an existing cellar conversion. For a full new excavation, some clients choose to relocate during the noisiest structural phase, though it is not always necessary depending on how the property is laid out.

Can a basement flood

A correctly specified and installed BS 8102 waterproofing system, including a sump and pump for cavity drain systems, is designed to manage groundwater indefinitely. This is a specification decision made at design stage, not something to value-engineer down later.

Does a basement conversion add value

Basement conversions in prime London postcodes commonly add £80,000 to £250,000 or more to a property’s value, though the return depends heavily on the local market and the quality of the finished space.

Get in touch to arrange a structural survey, and we will give you an honest assessment of what is possible beneath your property.