How Much Do Basement Conversions Cost?

If we look at current UK market prices for full basement conversions, including excavation, underpinning, drainage, waterproofing and all internal work, the average cost lands at around £1,000/m² for simpler conversions, and goes up to about £2,500/m² for full structural work.

London & South East: Costs lie at around £2,000–£3,000/m² for mid-range, complete conversions, but can rise to 3,500/m² or more for high-spec or complex builds.

If the work is limited to internal finishes only, such as painting, flooring, plastering and light renovation (with no structural changes), the cost may be half or less, typically around £750 to £1,200/m².

Where your project falls on this scale will depend on a range of factors. Some of these are within your control – like the type of flooring, tiles, sanitaryware and appliances you choose. Others are dictated by the property itself, such as existing ceiling height, soil conditions, water ingress and access.

Cosmetic Refurbishment vs Structural Work

This may seem obvious, but there is a huge difference in cost between simply redecorating a basement and having to excavate, underpin, waterproof and insulate it. If your basement already has enough ceiling height (typically around 2.2m finished), is dry, well insulated, and has enough windows for natural light and ventilation, your conversion will resemble a standard internal refurbishment – updating flooring, ceilings and walls.

However, if the basement is currently damp, dark and not fit for habitation, you’ll likely need to begin with substantial groundwork: underpinning foundations, installing waterproofing and drainage, pouring a new floor slab, cutting out new windows or lightwells, and improving headroom. This turns the job into a major construction project, significantly increasing the cost.

A Detailed Example

Below is an example of a mid range London-based basement conversion, consisting of a 40 square metre existing cellar that required increased ceiling height, waterproofing, and a full internal fit-out. All costs include both labour and materials:

Architect & structural engineer: £8,000

Underpinning & excavation to increase headroom: £35,000

Cavity drainage system (membrane, sump pump): £8,000

Concrete floor slab with insulation: £4,500

One lightwell and escape window: £6,000

Bathroom (mid-range spec): £6,500

Electricals, plumbing, and heating: £7,000

Stud walls, plastering, joinery, decoration: £14,000

Project management & contingency: £6,000

Total: £95,000

Cost per m²: Approx. £2,375

This is an example of a full conversion in London that includes structural work and a modern, but not luxury, finish. For simpler projects (. no underpinning, or fewer services), costs could be lower. Conversely, adding a kitchen, extra bathrooms, or high-spec finishes would push the budget higher.

Planning Your Basement Conversion

Every renovation project begins with a planning phase, and the more effort you invest here, the better your outcomes. A good plan prevents surprises that could derail the schedule or budget. Predictability and control are especially important in large projects like basement conversions.

Contacting an architect or basement contractor early is highly recommended. Not only can they advise on layout, structure and compliance, but they can also help you access skilled tradespeople, who may have limited availability. Planning ahead avoids delays.

Think carefully about what you want from the finished space. Will it be a self-contained flat? A family room? A gym? A home office? Your end goal informs everything from ventilation to layout, plumbing to power points. Early engagement with your contractor will help refine your design.

Planning permission

Usually, you won’t need planning permission for converting an existing basement into a living space. According to Planning Portal, planning permission “will be required where you:

• create a new unit for residential accommodation
• change the use significantly e.g. to operate a business
• add a lightwell
• alter the external appearance of the property.”

If your house is listed, you will likely need listed building consent, which is different from planning permission.

Remember to always check which rules apply to your specific basement conversion, and any local regulations or restrictions that may apply. When in doubt, ask. 

Building regulations

In all cases, you will need to comply with Building Regulations. These cover:

• fire escape routes
• ventilation
• ceiling height
• damp proofing
• electrical wiring
• water supplies

You will in all likelihood need building regulations approval for a basement conversion. A qualified architect or contractor can liaise with Building Control or an approved inspector on your behalf to submit plans, ensure compliance, arrange inspections and get the completion certificate. Some trades can self-certify their own work under competent person schemes, but the overall conversion will still need sign-off from building control and a completion certificate. 

Party Wall Act

If your basement conversion involves work on or near a shared wall with neighbors, you'll need a Party Wall Agreement under the Party Wall Act 1996. You must notify adjoining property owners at least two months before work begins. They can either consent or request a surveyor to create a legally binding Award outlining construction terms. This typically costs £1,000-£2,000 per neighboring property. While it's an additional expense, it prevents disputes and provides a framework for resolving issues during construction.

Finding a Safe and Reliable Contractor

A successful basement project depends on experienced, qualified professionals. Make sure you select builders with specific experience in basement conversions. Look for certifications and accreditations such as:

• Federation of Master Builders (FMB)
• TrustMark
• NICEIC or NAPIT 
• WaterSafe
• Fensa

Where possible, it is beneficial to choose contractors that belong to a competent person scheme, which means they can self-certify their work. It is also a quality assurance that they belong to such a scheme, which sets strict standards for membership. 

Ask contractors for references and ideally, examples of past basement work. A good builder should be able to explain how they manage waterproofing, ventilation, structural engineering and liaise with Building Control.

Don’t just go with the cheapest quote. Look for clear, detailed breakdowns and make sure each quote covers the same scope. Does it include materials? Waste removal? Building Control fees? Avoid vague estimates.

How to Lower Costs Without Lowering Standards

While basement conversions are major projects, there are ways to save money without cutting corners:

• DIY where appropriate: If you’re confident, you can handle minor work like painting or laying laminate floors. However, technical work is best left to professionals. You must not do gas work yourself - this must be done by a Gas Safe Engineer. Nor should you attempt anything related to structural, waterproofing and electrical work yourself. 
• Shop around for materials: Source tiles, flooring or sanitaryware yourself, and look for trade discounts or clearance deals.
• Reuse existing features: Keep the original staircase or leave parts of the basement unfinished (e.g. a utility or storage area) to finish later.
• Avoid scope creep: Stick to your original plan to avoid mid-project changes, which can quickly drive up costs.
• Compare multiple quotes: Always get at least three quotes and compare scope line by line.

Make sure that any cost-saving measures don’t compromise on essentials like waterproofing, structural safety, or fire escape compliance. These elements are non-negotiable.

What Needs to Be Done?

As we’ve seen, the cost of a basement conversion varies massively depending on the level of work involved. Some of the biggest price drivers include:

• Ceiling height: If it’s too low, you’ll likely need to dig down and underpin.
• External access: Adding a staircase, door or new entrance adds complexity and cost.
• Natural light: Creating new windows and lightwells is often essential for habitability.
• Ventilation: Mechanical systems may be needed to ensure airflow.
• Waterproofing and drainage challenges: High water tables or historic damp issues require robust systems.
• Bathrooms and kitchens: Conversions that include bathrooms or kitchens will also be more expensive due to the need for plumbing, ventilation, and electrical work.

If you’re lucky enough to have a basement that already has the right height, insulation, dry walls and adequate daylight, you’re in an excellent position to keep costs down. In that case, you need only worry about surface-level work such as painting, flooring, plastering and lighting. 

A Sound Investment?

Even if the cost of the project ends up being higher than expected, it can still make excellent financial sense. A quality basement conversion can add significant value to a home, especially in urban areas like London, where square footage is at a premium.

Additional liveable space – whether it’s an extra lounge, bedroom, or self-contained flat – can move your property into a higher price bracket. It also opens your home to a new market of buyers.

Creating a rental unit in the basement can provide passive income while increasing your home's overall value. Be sure to familiarise yourself with what requirements you must meet in order to legally rent out your property.

Even if you don’t rent it out, a second bathroom or WC in the basement can improve usability: It could be a worthwhile upgrade for busy households.

Get Started

Now you have a solid understanding of what a basement conversion in England entails – from typical costs to planning and execution. The next step is to get a few reputable contractors to assess your space and provide quotes.

When comparing quotes, make sure you’re looking at the full scope: does it include VAT, project management, structural work, waterproofing, decoration? A complete, transparent quote is the best starting point for a successful project.

With good planning, the right professionals, and an informed budget, converting your basement can be an exciting and transformative improvement to your home. Whether it’s for family, rental income, or resale value, it often proves a smart long-term investment.