Cellar Conversion Space You Already Own
Turning the existing cellar beneath your home into a dry, warm, usable room — a snug, cinema, gym, utility or guest suite — without the cost and upheaval of digging out a new basement. Proper waterproofing, head height where it can be won, light, ventilation and a full fit-out, designed and built by one team under one fixed-price contract.
Control
Converted
Weeks
No Hidden Cost-Plus
The Cheapest Room in the House to Gain
Many London houses sit on top of a cellar that is used for nothing but a boiler, some boxes and a bicycle. Yet it is a full floor of space you already own, already enclosed, already paid for — and converting it into a usable room is one of the most cost-effective ways to gain space in a city where space is everything. You are not digging downward into the clay or building outward into a garden; you are claiming a floor that is already there.
This page is for the cellar conversion — converting an existing cellar or lower-ground space into a dry, warm, genuinely usable room. It is distinct from a full basement conversion or basement excavation, where the floor is dug out or extended — a major engineering project with underpinning and a price to match. A cellar conversion works with what is there: waterproofing it properly, winning head height where it can be won, bringing in light and ventilation, and fitting it out to the standard of any room in the house. As a design-and-build contractor, we deliver the whole thing under one fixed-price contract.
The single thing that makes or breaks a cellar conversion is waterproofing. A cellar is below ground, surrounded by damp earth and, in much of London, a high water table sitting on clay. Convert it without a proper, guaranteed waterproofing system and you get exactly what you would expect — a damp, musty room that nobody uses and a buyer's surveyor flags. Do it correctly, with the right system, a maintained sump and a real guarantee, and the space is reliably dry for decades. This is where we are meticulous, and where cheap conversions fail.
The other key question is honest from the start: head height. A comfortable habitable room wants around 2.1m, and many cellars begin lower. Where it is just short, the floor can sometimes be lowered with underpinning to recover height; where it cannot, the cellar may still be perfect for a wine store, a gym, a plant room or a cinema. We tell you realistically what your cellar can become before you commit a penny.
We have converted cellars and lower-ground space across NW3, NW8, NW1, N6 and SW London — into media rooms, gyms, utilities, wine cellars and guest suites — each waterproofed to a guaranteed standard and finished to match the home above.
Six Uses for a Converted Cellar
A lower-ground room suits some uses better than others. These are the six we are asked for most, each shaping the specification.
Snug / Media Room
A cosy second living room or media space — the lower-ground level is naturally quiet and separate, ideal for a relaxed retreat away from the main living floors.
Home Cinema
The one room where no natural light is an advantage. With acoustic treatment, blackout and tiered seating, a converted cellar makes an outstanding home cinema, well away from the rest of the house.
Home Gym
A robust floor, good ventilation, mirrors and the power a gym needs — a dedicated workout space that keeps the equipment out of a bedroom and works even where head height is modest.
Wine Cellar / Store
The classic cellar use — a temperature-stable wine cellar or generous store. Lower head height is rarely an obstacle, making this the simplest and most affordable conversion of all.
Utility & Laundry
Moving the washing, drying, plant and storage downstairs frees valuable space upstairs — a hard-working utility floor that takes the clutter out of the kitchen and the rest of the house.
Guest Room / Office
With adequate head height, light and a means of escape, a converted cellar makes a guest bedroom with a shower room or a quiet home office — a genuine extra room where the rules allow.
What a Cellar Conversion Actually Includes
A dry, usable cellar is the sum of work you mostly cannot see. Every element below is delivered and warranted under one fixed-price agreement.
Survey & Damp Diagnosis
A proper assessment of the cellar — head height, structure, the water table, existing damp and the drainage options — so the waterproofing and conversion strategy is based on the real conditions, not guesswork.
Waterproof Tanking
A guaranteed waterproofing system — usually a cavity drainage membrane to walls and floor channelling water to a sump, or cementitious tanking — specified to BS 8102. The single most important element of the conversion, and the one we never compromise.
Head Height & Structure
Where extra height is needed, the floor lowered and walls underpinned in a controlled sequence to the structural engineer's design; where it is not, the structure simply made sound — the most economical route taken wherever possible.
Drainage & Sump Pump
A drainage channel feeding a sump with a reliable pump (and battery backup) to remove any water the membrane collects — the active half of a cavity drainage system, sized and maintained for the long term.
Insulation & Heating
Insulation to keep the lower-ground room warm and reduce condensation, and heating — underfloor or radiators extended from the house — sized for a space that loses heat to the surrounding ground.
Light & Light Wells
Bringing daylight in where possible — light wells, sun pipes or glazing to a lowered area — and a layered artificial lighting scheme that makes a below-ground room feel bright and welcoming rather than a basement.
Ventilation
Mechanical extract or whole-room ventilation to keep the air fresh and humidity controlled — essential below ground, and a key partner to the waterproofing in keeping the space dry and healthy.
Electrics & AV
Circuits certified to BS 7671, lighting, sockets, data and the AV pre-wiring a media room or cinema needs — all designed in behind the membrane so nothing penetrates the waterproofing later.
Bathroom / WC (Optional)
Where you want a shower room or WC — for a guest room or to make the floor self-sufficient — with the pumped drainage a below-ground bathroom requires, fully tanked like any of our wet rooms.
Flooring & Fit-Out
An insulated, level floor over the membrane, quality flooring, plastered or panelled walls, joinery and storage, and decoration to the standard of the house above — a finished room, not a tidied cellar.
Approvals & Guarantee
Building Regulations submission and any planning managed in-house, plus a manufacturer-backed waterproofing guarantee and our own warranty — the documents a future buyer's surveyor will want to see.
Snagging & Aftercare
A two-stage snag, commissioning of the sump and ventilation, a 12-month defects period, a 12-month defects period and a six-month service visit.
Is Your Cellar Right to Convert?
A handful of factors decide what your cellar can become, and what it will cost. We check all six at the survey, before any money is committed.
It Already Exists
The biggest cost saving of all — converting an existing cellar avoids the excavation, underpinning and engineering of a full basement dig-out. If a usable void is already there, you are most of the way home.
Head Height
Around 2.1m for a comfortable habitable room. Many cellars start lower; where it is just short, the floor can sometimes be lowered. We measure and advise honestly on what is achievable.
The Water Table
London's clay and high water table make waterproofing essential. We assess the conditions and specify a cavity drainage or tanking system to suit — the factor that most affects whether the room stays dry.
Drainage for a Sump
A cavity drainage system needs somewhere to pump the water it collects. We confirm the drainage route and design the sump and pump — with backup — for reliable, long-term operation.
Light & Escape
For a habitable room, natural light and a means of escape shape what is possible — and may need a light well, which can trigger planning. For a store or plant room, neither is an issue.
Planning Context
Conversion of an existing cellar is often Permitted Development; excavation, light wells or external changes, and any work to a listed building, usually need consent. We check your exact situation.
Eight Stages, From Brief to Handover
A below-ground conversion rewards a careful, well-sequenced process. Here is how we run yours.
Free Consultation
We discuss what you want the cellar to become, look at the space, and talk realistic budgets and what is feasible. No charge, no obligation.
Survey & Damp Diagnosis
A survey of head height, structure, the water table and damp, and an assessment of the waterproofing and head-height options — the basis for an honest design and price.
Layout & Waterproofing Strategy
The use, layout and waterproofing system designed and presented, with the head-height approach and any light wells resolved before you commit.
Building Regs & Planning
Building Regulations submission, and any planning application where excavation, a light well or external change is involved.
Underpinning & Lowering (If Needed)
Where head height is being won, controlled underpinning to the engineer's design and the floor lowered — the most involved phase, omitted entirely where it is not needed.
Tanking, Services & Fit-Out
The waterproofing system installed and the sump commissioned, then first fix, insulation, plaster, second fix, flooring and decoration. Weekly updates.
Commissioning & Handover
Sump and ventilation commissioned, a joint snagging walk, the certification pack and waterproofing guarantee, and handover.
Defects Period & Warranty
A 12-month defects period, a six-month service visit and a 12-month defects period, plus the waterproofing system's own guarantee.
Four Tiers, Honestly Drawn
What a cellar conversion costs depends on whether head height is being won and the level of finish. Here is how our four tiers differ.
Where Your Budget Actually Goes
For a representative conversion of an existing 22 sqm cellar, here is the honest breakdown of where every pound is spent.
The Engineering Beneath the Finish
A converted cellar stays dry and stable because of what is done to the structure and the waterproofing. Here is what is designed and installed.
Waterproofing
- Standard: Designed to BS 8102 (protection of below-ground structures)
- Type C: Cavity drainage membrane to walls and floor, draining to a sump — our usual choice in London
- Type A: Cementitious tanking where appropriate, applied to a sound structure
- Maintenance: Accessible sump and membrane for inspection and servicing
Drainage & Pumps
- Sump: Sealed sump chamber collecting water from the membrane
- Pump: Twin pumps with battery backup and high-level alarm for resilience
- Discharge: To a suitable drain or soakaway, designed and consented
- Foul: Pumped (macerator/sewage) drainage for any below-ground WC or shower
Structure
- Underpinning: Mass-concrete underpinning in a controlled sequence where the floor is lowered
- Calculations: Designed by MIStructE engineers to BS EN 1997 (Eurocode 7)
- Slab: New reinforced floor slab over the membrane and insulation
- Retained walls: Checked and made sound where they retain ground
Comfort & Compliance
- Insulation: Floor and wall insulation to limit heat loss and condensation
- Ventilation: Mechanical extract or whole-room ventilation for fresh, dry air
- Fire & escape: Means of escape and detection where the room is habitable
- Building Control: Full Plans submission, inspections and completion certificate
Planning & Building Regulations for Cellars
Converting an existing cellar is usually simpler on planning than a basement dig-out — but it always involves Building Regulations.
Converting an existing cellar into a habitable room generally does not require planning permission, because you are not changing the footprint or the external appearance of the house — it is treated as a change of use of existing space. This is a real advantage over a full basement conversion or excavation, which extends or creates the basement and almost always needs permission. What a cellar conversion always requires is Building Regulations approval, covering the structure, waterproofing, ventilation, fire safety and means of escape, and any drainage.
Planning permission does come into the picture if you excavate to win significant head height beyond the existing structure, add an external light well or new external access, or alter the appearance of the house — and Listed Building Consent applies to any work to a listed property. A light well is the most common trigger, because it changes the external ground and elevation. We assess exactly what your project needs at the survey and prepare and manage any application required.
We handle the Building Regulations submission, inspections and completion certificate as standard, and provide the manufacturer-backed waterproofing guarantee that, alongside our own warranty, gives you and a future buyer's surveyor confidence the room will stay dry. For an early budget, use the cost calculator; for a full dig-out instead, see basement conversions.
A Cellar Conversion, Week by Week
A representative programme for converting an existing cellar with adequate head height. Lowering the floor adds to this, but this is the core sequence.
What Sets Our Cellar Conversions Apart
Below ground, the margin between a dry room and a damp one is all in the detail. Here is what ours gives you.
Waterproofing First
A guaranteed, BS 8102 waterproofing system designed for London's water table — the element we never compromise.
Honest About Head Height
We tell you what your cellar can realistically become before you commit — and recommend the cheapest route that meets your brief.
Engineer In-House
Our MIStructE engineers design any underpinning and lowering — structure and build under one roof.
One Fixed Price
Survey, design, waterproofing, structure and fit-out under one contract — no cost-plus surprises below ground.
Directly-Employed Trades
Our core trades are PAYE staff — continuity and care on a technically demanding job.
£5M public liability, £10M employers' liability and contractors' all risks cover
Certificates are available on request.
Guaranteed Dry
A manufacturer-backed waterproofing guarantee alongside our own warranty — reassurance for you and any future buyer.
12-Month Warranty
A 12-month defects period covering workmanship from practical completion.
Explore Related Below-Ground & Space Work
A cellar conversion is one way to gain space. Start here, or speak to us about the alternatives.
Basement Conversions
Full basement conversion, including excavation and enlargement.
Basement Excavation
Digging out or down to create or enlarge a basement.
Basement Waterproofing
Tanking and cavity drainage systems for below-ground spaces.
Garage Conversions
Another existing structure converted to usable space at low cost.
House Renovation
A cellar conversion as part of a wider whole-house renovation.
Granny Annexe
A lower-ground level converted into self-contained accommodation.
Below-Ground Proof
Comparable projects with real lower-ground space. Browse the case-study hub and the wider portfolio, and for survey-led due diligence before you commit, see surveying support.
Belsize Park Gardens, NW3
Five-storey period house with lower-ground works as part of a whole-house programme.
View case study →Basement Conversions
The full dig-out and enlargement service where a cellar is not enough.
Explore →Basement Waterproofing
The tanking and cavity drainage that keeps any below-ground room dry.
Explore →More Completed Projects
Browse the full portfolio of conversions and renovations across London.
View portfolio →Detailed Answers to the Questions That Matter
What is a cellar conversion, and how is it different from a basement conversion?
How much does a cellar conversion cost in London in 2026?
Do I need planning permission for a cellar conversion?
How do you stop a converted cellar from being damp?
My cellar has low head height — can it still be converted?
How long does a cellar conversion take?
What can I use a converted cellar for?
Do you need to underpin or lower the floor?
How do you get light and ventilation into a cellar?
Will a cellar conversion add value to my home?
What Our Clients Say
“I would like to thank Ross and his team for their consistent commitment to quality and their unerring reliability. They delivered our property to specification and on time, proving to be an extremely effective, experienced, and proactive contractor.”
“We have worked with Ross and his company many times. They are extremely professional and hardworking individuals who can work under any circumstances. There was no variation to the works.”
Begin Your Cellar Conversion
Book a no-obligation consultation at our Finchley Road design studio or in your home. The first meeting is free, lasts 60–90 minutes, and concludes with an honest indication of feasibility, programme and budget band. No salespeople. No pressure.
Free Cellar Conversion Consultation
Site visit · feasibility assessment · outline cost estimate · programme indication. No obligation. Saturday appointments available.
Completed renovation project examples
