Basement Conversions in London

Why choose a basement conversion? Understanding the basement conversion process. 

A basement is a great option to add space to your house. When extending upwards or outwards is not possible, a basement conversion is perfect for adding living space. In London especially, where space is an extremely valuable asset, converting a basement can create that extra room that can really change your family life.

Technology and materials have evolved so that a basement is no longer a dark low-ceilinged space, they can be as light, broad and dry as above-ground rooms. Waterproofing and damp- proofing basements are a painless and effective procedure when you rely on the right professionals. If well designed and properly built, a basement can be used for virtually any purpose in your house, including an open plan kitchen, for example.

At houseUP we love building basements in London and we’re not shy to say that somehow we always manage to exceed our clients’ expectations. Thanks to our unique approach, home-life remains relaxed and stress-free during construction. There will always be a structural engineer in charge, to ensure maximum safety and security on site, as well as the temporary structural support that is required when building a basement.



  • Meet in person to discuss what you want and the challenges we might be facing.
  • View the property to place the vision within the physical constraints of your house.
  • Collect details and impressions and explore the neighbourhood so that we can understand the context of your home.
  • Discuss your ideas to better identify challenges, anticipate problems, and foresee potential obstacles.

When creating or converting a basement under an existing building, and even creating a basement for a new build, it is important to understand the physical constraints. A lot of crucial information can be gained from the council planning authority, but a site visit always reveals more than any document can do.

Many elements can affect the feasibility of a basement, including party walls, layout, nature of the soil, and underground structures like road tunnels or Tube networks. Also a visit to the neighbourhood will give us an idea of whether there are similar developments in the area, which would mean it’s more likely to get planning permission. If you are converting an existing cellar for example, it is possible your basement conversion will fall under permitted development work and you won’t need to apply for planning permission.

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  • Evaluate all the physical factors that will affect the conversion of your basement. We take into account the load of the building above, the nature of the soil, the water table level, the underlying existing structures like services, tunnels, Tube lines etc.
  • Determine the formal process, which means examining the legal aspects and studying the council regulations; investigating the layout of the neighbourhood and the adjacent properties; defining the need for party wall agreements, external technical reports, method statements etc.
  • Make informed decisions relating to: the value added by building the basement, against the budget investment, and when all the feasibility constraints are clear, choosing the best options for structure, design, usage, dimensions, layout etc.
  • Finalise the pre-construction phase by producing all the documentation including technical drawings, structural calculations, the method statement, work plan, and submitting the plans to the council for permitted development or planning permission, serving the party wall notices and completing the party wall agreement.
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  • Define the costs and evaluate the basic construction work.
  • Estimate the options and optimise the choice of finishes and suppliers for the best result.
  • Communicate the quote clearly regarding what is included and what is not and what the impact of that might have.
  • Explain the process and the program, define the communication channels, and agree on the decision flow.

Investing your money in renovating your property can be a big decision. That is why understanding the quotation is so important. Ultimately, you want the price to be reasonable and reflect quality. houseUP will never quote unrealistically low or high prices, we will be transparent in any unavoidable additions, and we will always complete the project.

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  • Enforce the method statement so that all employees are informed and safe. Clear and concise instructions will be created for the team to carry out the work. These include accurate descriptions and precise details such as props for vertical and lateral support, temporary roof, usage of conveyor belts for soil removal etc.
  • Appoint experienced and trustworthy teams, i.e.: basement specialists and the a project manager with specific expertise.
  • Supervise the site and stay involved with day-to-day activities in order to provide guidance, coordinate the personnel, the supplies, and the procedures on-site.
  • Report and share information, give regular feedback and respond promptly to questions and requests.
  • Set up the communication channels: dashboard, live chat, photo and video sharing using the latest tech solutions.
  • Make timely decisions and follow a clear process for requests, approvals, and last-minute changes.

Building a basement requires technical competence and experience, and in our experience basement contractors work better under the supervision of professional experts.

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  • Athough the requirements on the finishes are often defined in the design phase, it is important all the details are approved by you. Windows, flooring, lighting, doors, paint, tiles, and fittings are all details that make a difference to the quality of a basement.
  • We choose the right tradespeople for all the work on site,; electrical and plumbing, for example, are crucial to a successful, long-lasting structure. We appoint the best teams so that future issues are prevented. houseUP ensures you get the best specialists for electrics, plumbing, tiling, painting, flooring, decoration etc.
  • We will make sure all the finer details that need to be retouched are taken care of before hand over.

While building the shell of your basement is very important, you also want to make sure that the space you create is beautiful and comfortable. Therefore, the finishes are essential. We will make sure you are involved and informed so that you can choose the most suitable finishes for your lifestyle and living space at top-notch quality.

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    • Exceptional designing and planning that maximises the potential of your house.
    • Planning permission sorted.
    • Building regulations approved and the work started in a timely manner.
    • (in-house) Technical competence and experience of an in-house team who specialise in basement conversions.
    • (in-house) All the documentation specific to basements, including method statements, prepared.
  • 02BUILD 
    • Specialist building contractor teams with extensive basement experience.
    • A structural expert on-site to double-check current conditions of the house and to ensure safety and compliance.
    • The best tradespeople from electricians and plumbers to carpenters, decorators and more.
    • Access to exclusive kitchens and porcelain tiles direct from Italy at discounted prices.
    • Your vision to be realised with the quality of details meeting your expectations.
    • A dedicated project manager and a structural engineer who specialise in basement conversions.
    • Prompt and professional handling of Health and Safety, party wall procedures, planning authorities, and sensitive approach to neighbours.
    • Efficient time management relating to orders, personnel on site, building regulations visits and other arrangements and appointments.
    • A clear decision making process that maintains the running of the site.
    • Control of the houseUP dashboard, to monitor progress, budget consumption, see real- time photos and videos, and make comments.
    • Full involvement in the process, the schedule, abd the choice of materials and finishes, etc.
    • Access to a dedicated channel on the houseUP chat platform, for fast, trackable decision-making and issue management.
    • A centralised, written record of all the conversations, questions, issues, and decisions made, that eliminates lengthy email thread discussions.

Tanking and Waterproofing 

Tanking is a building technique for underground structures that controls the water that comes from the external soil. By using draining channels to remove excess water, the basement is protected from infiltrations by a tank that surrounds the walls and the floor of the basement. Cavity drain membranes, channel drains and sump pumps are employed to remove the excess water. The pumps are installed with redundancy and alarms. Secondary pumps can take over when the primary is full or not operating properly, while the alarms signal issues in the correct functioning, like battery exhaustion.

Waterproofing is a more generic term for ensuring that a structure, underground or overground, is not infiltrated by water, causing damp. For basements, waterproofing membranes are added to the tanking, to prevent water penetrating the basement.

At houseUP we have the in-house expertise to design your basement and implement the latest technologies in waterproofing systems. This combination will ensure your basement lasts longer.

Tanking and Waterproofing


Having an idea of how much a basement conversion costs as an average will help with budgeting. However, it’s important to always keep in mind that there are variables that can both add to and lower the average cost. houseUP will support you in making choices that balance your budget requirements and your vision.

Baseline costs of construction in London, which don’t include professional fees and finishes like flooring, windows, doors, furnishing and joinery, fittings etc:

  • Creation of a new basement under an existing building: £3,000 to £5,000 per sqm.
  • Conversion of an existing cellar: £2,250 to £3,750 per sqm.
  • Extension of an existing basement/cellar to the garden: £2,500 to £4,000 per sqm.


Questions and answers

Collected questions that we often hear from you

  • How much will my refurbishment cost? 

    Every good building company will tell you that it is not possible to estimate the cost of refurbishment for a flat or a house before all the details have been defined. But we understand that it is important to have a reference figure before embarking in the process for budgeting purposes.

    We suggest a conservative estimate of £1,750 per square metre as the absolute minimum, and an average of between £2,000 and £3,500 per square metre.

    However, it’s important to keep in mind that there are details that can impact the cost significantly, the most costly being basement excavations or extensions of existing lower ground spaces; inclusion of very large or high spec glazing; very high energy performance materials.

    Additionally, some finishes can increase costs significantly, like paint, tiles or flooring, especially if the surfaces are very large,.

    In general, kitchens can be installed from as low as a few thousand pounds to hundreds of thousands, too.

  • How long will my refurbishment take? 

    As per the duration of a project, it cannot be estimated without the full details. If you want to plan ahead, for alternative accommodation for example, we recommend at least:

    For smaller flat refurbishment, approximately 4 to 6 months. For house refurbishment, approximately 6 – 8 months These estimations relate to straightforward refurbishments like loft conversions or rear and side extensions. More complex work like basement excavations can only be estimated with more details, especially if access to the property is a consideration. Composition of the soil etc are also important factors.

  • Can I live at the property during the refurbishment? 

    We always recommend that you arrange for alternative accommodation during the project.

    Some of our clients choose to live at the property during the work, and, of course, we make that possible. However, please be advised that this arrangement comes with a significant increases in stress, cost and duration of the project.

  • I want to do everything with one company, can you do design and refurbishment for me? 

    Yes we can. We partner with the best London architects to create designs that suit you and your needs. We will match you with architects who have specific experience in your area, share your vision and style for your refurbishment project.

    Doing the design and build with us gives you a view and control of the projected costs of your refurbishment from the very first day. You will be able to make well-informed choices for your design while understanding the cost implications and opting for the best solution for your investment.

    Check out our Design & Build page for more information.

  • I have all the drawings ready with my architect, can you do the building works for me? 

    We certainly can! We work with the best architects in London through a tender process, for clients that choose to have the designs for their refurbishment made by an independent architect.

    We can provide quotes in our own very detailed format or in your architect’s preferred format.

  • Do I need planning permission for my refurbishment? 

    You won’t need planning permission for most refurbishment projects. If you are not changing the external appearance of your flat or your house, you won’t need planning permission. In many cases, you can make a loft conversion or a rear/side extension within permitted development rights. You can find all the details on which type of extensions are allowed under permitted development here.

    However, if you are planning to refurbish a listed building, you may need permission.

  • What works make the biggest difference in the final cost of my refurbishment project? 

    Typically excavation to extend or create a basement under an existing property is one of the most expensive types of work in construction. The cost of finishes is another big factor in the final cost of a flat or house refurbishment.

  • I am buying a property, can you help me estimate how much the refurbishment would cost? 

    When you buy a new property, it is very likely that you’ll plan for a refurbishment. You can find some very good deals on the market, where you can buy a property for a good price when it needs some work done.

    By offering an estimation on work to be done, we can help you make the right decision and offer the right price for the potential property.

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Get in touch with our basement specialist team

If you are considering a basement conversion in your property we can answer all your questions, including feasibility, planning permission, technical aspects and estimate costs

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