Basements and cellar conversions
What does basement excavation cost?
Lowering an existing cellar and digging a new box under a terraced house are not the same job, and the gap between them is the whole story. Depth, spoil removal through a house, the waterproofing system, the temporary works and the party wall process are what a basement contractor is actually pricing. Below is what moves each of those, and what to put in writing so two quotes can be compared. Tell us about the property and basement contractors will price your job.
3m and 6m
the two distances from a neighbour's building that trigger a party wall excavation notice
1 month
minimum notice before beginning to excavate where section 6 applies
8 drivers
the decisions that account for most of the spread between basement quotes
What happens next
- Tell us about the property: what you want dug, how spoil gets out, how close the neighbours are. Two minutes, no account.
- We pass your details to basement contractors covering your postcode, and to no one else.
- They price your actual job, almost always after a site visit. Compare, choose, or walk away.
Basement Excavation Cost is a free introduction service operated by Ellul Solutions Ltd. We are not a contractor, a structural engineer, a waterproofing designer or a party wall surveyor, and nothing here is structural, legal or planning advice. We are not affiliated with any government department, local authority or standards body named on this site. We pass your details to relevant local basement contractors who quote you directly; we may receive a fee from providers, you pay nothing and are under no obligation. Statutory points are cited to legislation.gov.uk and gov.uk and were checked on the date shown.
What sets the price of a basement excavation, 2026
Last updated
We do not publish a headline rate per square metre for basements, because the figures that circulate are so wide that they mislead more than they help. A cellar conversion with the floor left alone and a full dig-down and box under a Victorian terrace are different engineering problems, and the same square metre of finished floor can cost several times more in one than the other. This table is what basement contractors actually price, why each line moves the number, and the wording that forces the item into a quote.
This is a cost driver table, not a price table. Each row names a decision or a site condition that basement contractors price separately, states the direction it moves a quote, and gives the exact thing to ask for so competing quotes cover the same scope. It carries no pound figures because the honest range for a basement is too wide to be useful, and because the two biggest variables, ground conditions and access for spoil removal, are site-specific and cannot be estimated remotely. Statutory points are cited to legislation.gov.uk and gov.uk. For a real number, get quotes on your actual property.
| Cost driver | Why it moves the price | What to ask for in the quote |
|---|---|---|
| Method: convert, lower, or dig new | Converting an existing dry cellar is a fit-out job. Lowering the floor means underpinning the existing walls in sequence. Digging a new box under an occupied house is a temporary works exercise with propping, sequencing and engineering supervision. These are three different trades' worth of work sold under one word. | Which of the three the price assumes, in writing, and what happens to the price if survey shows the existing structure cannot take the assumed method. |
| Depth, and therefore volume of spoil | Every extra 300mm of dig is more spoil, more underpin depth, more retained height and more temporary works. Cost does not scale politely with headroom, it steps as depth pushes the design into heavier propping. | The finished floor-to-ceiling height the price assumes, and a cost per extra 100mm of depth. |
| Access and how spoil leaves the site | The single biggest London and inner-city variable. Where a machine can reach the dig, muck away is efficient. Where every barrow goes through the hallway or up a conveyor to a lorry on a suspended parking bay, the programme lengthens and labour dominates. Skip and lorry permits, wheel washing and restricted hours all attach here. | How spoil is assumed to leave, how many lorry movements are allowed for, and who obtains and pays for permits and suspensions. |
| Underpinning and temporary works design | Underpinning in sequenced pins, needle-and-prop schemes and piled or contiguous walls are all valid answers with very different prices. The temporary works design is a professional service, not a contractor's freebie, and on a party wall job the adjoining owner's surveyor will want to see it. | Who designs the temporary works, whether that fee is in the price, and whether the structural engineer's site inspections are included. |
| Waterproofing system and its maintenance | British Standard practice for below-ground waterproofing distinguishes barrier protection (often called Type A tanking), structurally integral protection (Type B) and drained cavity protection (Type C). A cavity drain system brings sump pumps, a power supply, backup and a maintenance regime; a tanked system depends on workmanship it is hard to inspect later. The choice, and whether two systems are combined, moves both price and lifetime cost. | The system type, who the waterproofing designer is, the guarantee and what maintenance the guarantee requires you to carry out. |
| Ground conditions and water | Made ground, running sand, a high water table, an old well or a lost river changes everything from the propping design to the pumping strategy. Nobody can price this reliably without investigation, so a quote given without it is carrying risk that eventually lands with you. | Whether a site investigation or trial pits are included, and exactly who carries the cost if ground conditions differ from the assumption. |
| Party wall process and the neighbours | Excavating within 3 metres of a neighbour's building and below the level of the bottom of their foundations, or within 6 metres where the 45 degree line applies, brings the Party Wall etc. Act 1996 into play, with at least one month's notice before you begin. The Act also lets the adjoining owner require you to underpin or otherwise safeguard their foundations at your expense. | Whether party wall surveyor fees, schedules of condition, and any strengthening required by an adjoining owner are inside or outside the contract price. |
| Fit-out, light and services | A finished basement needs stairs, ventilation, escape provision, drainage that pumps upward, heating, lighting and often a lightwell to bring daylight in. These are what turn a dry box into usable rooms and they are frequently priced separately from the shell. | A separate priced schedule for stairs, mechanical ventilation, pumped drainage, electrical installation, lightwell or sunken courtyard, and the internal finishes. |
- Basement quotes are only comparable when the method (convert, lower, or dig new), the spoil removal route, the waterproofing system and the temporary works design fee are each stated separately.
- Under section 6 of the Party Wall etc. Act 1996, a building owner proposing to excavate within 3 metres of a neighbouring building and below the level of the bottom of its foundations must give notice at least one month before beginning to excavate; a 6 metre rule applies where a 45 degree line from the neighbour's foundations meets the excavation.
- Section 6(3) of the Party Wall etc. Act 1996 provides that the building owner may, and if required by the adjoining owner shall, at his own expense underpin or otherwise strengthen or safeguard the foundations of the adjoining owner's building so far as may be necessary.
Cite this page
“What sets the price of a basement excavation, 2026”, Basement Excavation Cost, https://basementexcavationcost.co.uk/ (updated 2026-08-14). This is a cost driver table, not a price table. Each row names a decision or a site condition that basement contractors price separately, states the direction it moves a quote, and gives the exact thing to ask for so competing quotes cover the same scope. It carries no pound figures because the honest range for a basement is too wide to be useful, and because the two biggest variables, ground conditions and access for spoil removal, are site-specific and cannot be estimated remotely. Statutory points are cited to legislation.gov.uk and gov.uk. For a real number, get quotes on your actual property.
Worth knowing
Every figure sourced and dated.
Basement conversion cost: what actually changes the number
Basement conversion cost depends on whether you convert, lower the floor or dig new. What each method involves, where the money goes, and how to get comparable quotes.
Party wall notice for a basement: the 3m and 6m rules
Excavating for a basement within 3m or 6m of a neighbour's building triggers the Party Wall etc. Act 1996: one month's notice, plans, and a duty to safeguard foundations.
Basement waterproofing: tanking vs cavity drain systems
Barrier tanking, integral protection and drained cavity systems compared for a basement: how each fails, what maintenance each needs, and what to ask a specialist.
Basement planning permission and building regulations
Do you need planning permission for a basement, and what building regulations approval covers: structure, escape, ventilation, drainage and the right sequence.
Straight answers
How much does it cost to dig out a basement?
There is no single credible figure. The cost turns on the method (converting a dry cellar, lowering an existing floor, or digging a new box), the depth, how spoil leaves the site, the ground and water conditions, and the waterproofing system. Restricted-access city sites, London in particular, sit well above the rest of the country for identical work. Fix the scope in writing, then get three quotes on it.
What is the most expensive part of a basement project?
On a restricted-access site, excavation and spoil removal, because everything has to be handled by labour rather than machines. On an open site, the structural and temporary works package usually leads. Waterproofing is rarely the biggest line but it is the one most likely to cost you again later if it is specified cheaply.
Do I need a structural engineer for a basement?
Yes. Underpinning, retaining structure and the temporary works that support the house while you dig all need designing by a structural engineer, and building control will expect calculations. Treat any contractor who offers to do it without engineering design as a serious warning sign.
How long does a basement excavation take?
Longer than the dig itself suggests, because the sequence matters more than the volume. Site investigation, structural and waterproofing design, planning where needed, building regulations, and at least one month of party wall notice before excavation begins all come first. Sequenced underpinning is deliberately slow work, and restricted access lengthens it further.
Will a basement add more value than it costs?
Not reliably, and we will not pretend otherwise. Digging out new basement space is normally the most expensive way to add floor area to a house, and whether the finished value covers it depends entirely on local values per square metre. Basements make sense where there is no alternative way to gain space, and the arithmetic should be checked against local sale prices before you commit.
Is this quote service really free?
Yes, free to you and with no obligation to accept any quote. We pass your details only to relevant local basement contractors, who quote you directly. We may receive a fee from those providers, which is how the service is funded.