NOTE Nº 0667 · REV B

Build Basement Cost: 2026 UK Price Guide

UK build basement costs in 2026: new-build basement, underpinning, tanking, and basement conversion prices per m² with full cost breakdown.

By The Renovation Notes Team · 7 min read · Published 6 September 2026 · updated within the last month · Sources checked 30 September 2026

Empty basement with exposed wooden beams, concrete walls and stairs.

In short

Building a new basement in the UK costs £2,500–£4,500 per m² of floor area, or roughly £100,000–£270,000 for a typical 40–60 m² footprint. Converting an existing cellar to habitable use costs £15,000–£50,000 depending on waterproofing and condition.

Building a basement is the most capital-intensive home improvement project most UK homeowners will ever undertake. In London in particular, where extending upward and outward is constrained by planning rules and plot size, downward extension has become the high-value solution for gaining significant floor area. The cost reflects the complexity: a new basement involves specialist excavation, structural engineering, waterproofing, and significant programme risk.

In 2026, a new-build basement under an existing house typically costs £2,500–£4,500 per m² of usable floor area, so a 45 m² basement runs roughly £112,500–£202,500; the itemised table below, which also counts surveys, fees and party wall costs, spans £103,000–£264,000 because site conditions vary so widely. Converting an existing unfinished cellar to habitable use (where the shell already exists) is significantly cheaper at £15,000–£50,000 for a cellar of 25–50 m².

Types of Basement Project

There are three distinct types of project, each with different cost profiles:

1. New basement excavation under an existing house, the most expensive and disruptive. Involves excavating beneath the existing footprint, usually by underpinning the existing foundations and removing soil through the house or via a temporary external opening.

2. New basement excavation as part of a new build, far cheaper per m² when built as part of a new structure, since the excavation happens before construction above-ground. Typical cost: £1,200–£2,000/m² when included in a new-build project.

3. Cellar or basement conversion, converting an existing shell from storage to habitable use. Requires structural assessment, waterproofing (tanking or cavity drain membrane), insulation, and services. Typical cost: £15,000–£50,000 for a 25–50 m² cellar.

Cost Breakdown: New Basement Under Existing House

Every line is for one basement of about 45 m² of usable floor area, which is the size the total at the foot of the table assumes:

Work element	Typical cost	Notes
Structural engineering and surveys	£5,000–£15,000	Includes geo-technical survey, structural design, CCTV drainage survey
Planning / Building Regs fees	£2,000–£5,000	Building Regs required; planning permission often needed in conservation areas
Party wall surveyor (terraced/semi)	£2,000–£6,000	Notice needed if excavating within 3 m of their building to below its foundations, or within 6 m and cutting a 45° line from those foundations
Excavation and soil removal	£20,000–£50,000	Highly variable, access, soil type, volume
Underpinning existing foundations	£15,000–£40,000	Traditional mass concrete underpinning or mini-pile system
Reinforced concrete structure	£25,000–£60,000	Walls, floor slab, roof slab
Waterproofing (Grade 3 system)	£8,000–£20,000	Cavity drain membrane + sump pump, or tanking + cavity drain
Insulation (walls, floor, roof slab)	£3,000–£8,000	Rigid foam or spray foam depending on system
Structural opening (stairs, lightwells)	£5,000–£15,000	Includes steelwork and lightwell construction
First fix M&E (electrics, plumbing, ventilation)	£8,000–£20,000	Including sump pump electrics, MVHR
Second fix and finishes	£10,000–£25,000	Plasterboard, floor finish, painted walls
Total (45 m² basement)	£103,000–£264,000	Wide range reflects site complexity

Section 6 of the 1996 Party Wall Act requires notice where you excavate within 3 m of a neighbour’s building to below the bottom of its foundations, or within 6 m where the excavation would cut a 45° line drawn down from those foundations.¹

These figures exclude VAT, which is charged at the standard 20% rate on work to an existing home.² The 5% reduced rate applies to conversions that change the number of dwellings, or to homes empty for two years, so a cellar conversion inside an occupied house does not qualify.²

Waterproofing Systems (BS 8102)

Building Regulations Approved Document C (requirement C2, resistance to moisture) and BS 8102:2022 (protection of below ground structures against water ingress) govern basement waterproofing.³⁴ The standard identifies three forms of protection:⁵

Type A (barrier protection): Tanking, applied to the outside or inside of the structure. External tanking is best practice but inaccessible on retrofits; internal tanking is cheaper but risks hydrostatic pressure forcing the membrane off over time.
Type B (structurally integral protection): Waterproof concrete (WPC), the concrete structure itself is designed to be watertight, with carefully designed joints. Requires specialist mix design and supervision.
Type C (drained protection): Cavity drain membrane, a dimple-profile membrane creates a drainage channel inside the basement wall and floor, directing water to a sump pit with a submersible pump. It is the system most often used when retrofitting a basement.

Habitable basements should be designed to Grade 3 under BS 8102, where no water ingress or damp areas are acceptable, and designers often combine two forms of protection, such as Type A with Type C.⁶ Sump pump systems cost £1,200–£3,000 for a duplex (twin-pump, battery-backup) unit, the standard for habitable use because failure of a single pump in a heavy rainfall event can flood the space within hours.

Cellar Conversion Costs

The lines given per m² are rates for the surface named. The lump sums are for one cellar of about 30 m², which is the size the total assumes:

Element	Cost range
Structural assessment and damp survey	£800–£2,000
Cavity drain membrane system (installed)	£80–£150/m² of wall + floor
Sump and pump (duplex unit)	£1,200–£3,000
Insulation (rigid foam to floor and walls)	£20–£45/m²
Partitioning, plasterboard, plastering	£30–£60/m²
Floor finish (screed + tile or LVT)	£40–£80/m²
First and second fix electrics and lighting	£3,000–£8,000
Ventilation (MVHR or extract)	£800–£2,500
Total for 30 m² cellar (about 80 m² of wall and floor)	£16,000–£36,000

Planning Permission

Whether a basement project needs planning permission in England depends on what you are doing:

Excavating a new basement is likely to require planning permission because of the major works involved.⁷
Converting an existing cellar to living space is unlikely to require permission, provided the external appearance does not change.⁷
Lightwells alter the external appearance and do require consent; a front lightwell visible from the street is judged more strictly than one at the rear or side.⁷
Local planning authorities may set their own basement rules in supplementary planning documents, which can limit a basement’s depth and size.⁷ Permitted development rights can also be withdrawn by an Article 4 direction, so check locally before assuming anything.⁸

Consult a planning consultant or submit a pre-application enquiry before committing to any expenditure. Budget £3,000–£8,000 for planning consultant fees on a complex basement application.

Programme

A new basement project under an existing house typically takes 6–12 months from planning approval to completion, of which structural works alone take 3–5 months. During excavation and concrete works, the house is usually habitable above, though access through certain ground-floor rooms will be restricted.

Is It Worth It?

Even in prime London locations, a basement often adds less to the value of the property than it costs to build, which makes it a poor pure investment but a high-value lifestyle addition. In regional cities, the payback is lower. The valuation test is a simple one: subterranean additions add value only where comparable sold prices demonstrate that buyers pay a premium for the additional square footage.

Get at least three quotes from specialist basement contractors (not general builders), and verify their experience with at least two or three comparable completed projects with references you can contact.

References (8)

Figures in this note were checked against the sources below on 30 September 2026. Superscript numbers in the text point to them. Every source the site cites, by topic.

1technicalvery strongThe National Archives (legislation.gov.uk), legislation.RN-QBUTY2The exact wording of the two excavation triggers that decide whether a basement or a deep foundation needs a party wall notice, worth quoting rather than paraphrasing. Section 6(1) applies where the building owner proposes to excavate, or excavate for and erect a building or structure, within a distance of 3 metres measured horizontally from any part of a building or structure of an adjoining owner, and the excavation will within those 3 metres extend to a lower level than the level of the bottom of the neighbour's foundations. Section 6(2) applies within 6 metres measured horizontally where the work will within those 6 metres meet a plane drawn downwards in the direction of the excavation at an angle of 45 degrees to the horizontal. Section 6(5) requires the building owner to serve notice on the adjoining owner at least one month before beginning to excavate. Section 6(7) provides that if the adjoining owner does not serve a notice indicating consent within 14 days, they are deemed to have dissented and a dispute has arisen, which is what triggers the surveyor and award process.
2technicalvery strongHM Revenue & Customs, guidance.RN-KBNHMBHMRC's main VAT notice for building work, last updated 26 August 2026. Sets out that construction of a new building and work to an existing building is normally standard rated at 20%, with exceptions: 0% for new qualifying dwellings and for housing association conversions of non-residential buildings, and 5% for a 'changed number of dwellings' conversion (section 7) and for renovation or alteration of a dwelling not lived in during the 2 years immediately before work starts (section 8, conditions at 8.1.2 and 8.3). Cites the law: VAT Act 1994 section 30 and Schedule 8 Group 5 for zero rating, section 29A and Schedule 7A Groups 6 and 7 for the reduced rate. Also explains the developer input tax block on goods that are not building materials (VAT (Input Tax) Order 1992 articles 2 and 6) and points self builders to the DIY housebuilders refund scheme.
3technicalstrongBSI.RN-UTBMKQThe current edition of the UK waterproofing code, published 31 March 2022, superseding BS 8102:2009. It gives recommendations and guidance on dealing with and preventing the entry of water from external sources into structures partly or wholly below ground, and it recognises three types of protection which a specification must name: Type A waterproofing barrier materials applied to the structure, Type B structurally integral watertight construction, and Type C drained cavity construction. It requires the waterproofing strategy to be set at the earliest planning and design stage, covering structural design, overall weatherproofing design, waterproofing design, construction sequencing and buildability, and it treats external drainage and openings as part of the strategy rather than as details. It also sets out evaluation of groundwater conditions, consideration of harmful ground gases, and the risk assessment that drives the choice of grade. Descriptors include basements, tanking, vapour barriers, render, ground-water drainage and site investigations. This is the standard to cite for a basement shower room or a below-ground floor build-up.
4technicalvery strongMinistry of Housing, Communities and Local Government.RN-NPO8FJThe statutory figures behind a damp diagnosis, and the numbers to quote when a note explains why ground levels or a bridged damp proof course cause rising damp. Paragraph 5.2 requires that in an external wall the damp-proof course should be at least 150mm above the level of the adjoining ground (Diagram 8) unless part of the building protects the wall, and that in an external cavity wall the cavity should be taken down at least 225mm below the level of the lowest damp-proof course, or a damp-proof tray provided (Diagram 9a). For suspended timber ground floors it requires a ventilated air space of at least 75mm from the ground covering to the underside of any wall-plates and at least 150mm to the underside of the suspended timber floor or insulation, with ventilation openings on two opposing external walls of not less than 1,500mm2 per metre run of external wall or 500mm2 per square metre of floor area, whichever is greater; for suspended concrete floors the clear ventilated space is at least 150mm with the same opening sizes. Section 2.39 onwards covers radon, referring to BRE Report BR 211 for protective measures. The 2013 edition was published on 3 September 2013 with changes in effect from 1 October 2013.
5technicalgoodProperty Care Association, guidance.RN-AM9E78The Property Care Association's summary of BS 8102:2022, naming the three forms of waterproofing protection (Type A barrier, Type B integral, Type C drained) and noting they can be combined.
6technicalgoodProperty Care Association, guidance.RN-06PGFAThe PCA code of practice for existing basements, which says habitable space should be designed to BS 8102:2022 Grade 3, where no water ingress or damp areas are acceptable.
7technicalvery strongPlanning Portal, guidance.RN-P4FUV2The Planning Portal on planning permission for basements in England. Excavating beneath an existing house is often not development requiring permission, but a lightwell, a separate entrance, or a change to the external appearance usually is, and many London boroughs have specific basement policies. The source for why the answer here is more local than for most projects.
8technicalvery strongMinistry of Housing, Communities and Local Government (GOV.UK), guidance.RN-SM4299States that planning permission is only needed if the work is development within section 55 of the Town and Country Planning Act 1990, and that interior alterations and works that do not materially affect the external appearance of a building are not development (paragraph 001). Explains permitted development rights under the GPDO 2015 and their restriction on article 2(3) land such as conservation areas, National Parks, AONBs, the Broads and World Heritage Sites (paragraphs 016 to 017), prior approval including the neighbour consultation scheme for larger single-storey rear extensions (paragraphs 026 to 032), lawful development certificates (paragraph 024) and article 4 directions, which must be applied in a measured and targeted way to the smallest geographical area possible and can make an authority liable for compensation (paragraphs 036 to 043). Page last updated 26 July 2023.