Basement conversions in London

Why go for a basement conversion?

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 an additional living space. Especially in London, 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. Waterproofing and damp proofing basements are never an issue 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.

In houseUP we love building basements in London because somehow we always manage to surprise our clients with a better result than their expectations. With us they enjoy a relaxed and hassle-free life while the construction is in progress, thanks to our unique approach. We always have a structural engineer in charge, to ensure the maximum safety and security on site, including the temporary structural support that is required while building a basement.


What to do with a basement conversion

A basement is ideal to create more living space in a house in London, especially extra space for the family. One great option that a basement gives you is to build an independent entrance, so to create a self-contained unit.

Usually basements are converted into:

  • a play room for your children
  • a cinema room for the family and friends
  • a gym or a sauna
  • a utility room for appliances and laundry
  • a storage space for valuables like wine, cigars or other collections
  • a hobby lab for adults and kids
  • a home office with an independent entrance

Indeed, if a basement is well designed and built, it can be used for any purpose. One of our clients in London, for example, chose to have their open-plan kitchen living room in the basement of their 4 storey house in Greenwich.

Shell-and-core and fit-out

When converting a basement, it is very important to select the right basement builders, for each stage of the work. Working with houseUP, you will have access to a pool of basement builders that are specialised in basement conversions.

The advantage of hiring houseUP is to have a construction project manager who is also a structural engineer, so to ensure not only that the final structure of you basement is safe and solid, but also that the works are carried out in a safe and controlled process throughout.


In the first phase, you excavate to create or extend the volume for the basement. This includes temporary works to support the existing structure of the house while the basement works are in progress. The result will be a waterproof reinforced concrete box that constitutes the external structure of the basement.

It is important to employ a specialised subcontractor to carry out this phase of the project. houseUP only employs contractors that have a solid portfolio of damp proofing basements.

The added value of working with us is to have access to a pool of experienced and tested companies specialised in basement conversions; plus, only houseUP will always appoint a construction project manager who is also a structural engineer on-site to supervise the works during this very delicate phase.


The following phase is the fit-out. This phase includes electrical and plumbing work, wall creation, flooring, lighting, plastering and painting.

With houseUP, you don’t need to worry about finding the right tradespeople for each job. Our project manager will take care of everything and report to you for project status, schedule updates and decision making at the right moment. They will help you clarify your requirements and make your decision at the right moment, throughout the duration of the fit-out, until completion.


Here’s what to expect when doing your basement with houseUP


  • Make the most of the potential of your house with the best planning and designing expertise
  • Get planning permission sorted in a smooth hassle-free process
  • Have all the building regs approved to get the actual work started
  • (in-house) Use the technical competence and experience of a in-house team specialised in basement conversions
  • (in-house) Prepare all the documentation specific for basements, including method statements


  • Get the best specialised building contractor team with basement experience
  • Have a structural expert on-site to double check current conditions of the house and to increase safety and compliance
  • Use the best tradespeople on site at the right time, from electricians and plumbers to carpenters, decorators and more
  • Access exclusive kitchens and porcelain tiles directly from Italy at discounted prices
  • Achieve what you want with the quality of details up to your expectations

Project management

  • Have a dedicated project manager who is a structural engineer specialised in basement conversions
  • Ensure Health and Safety, party wall procedure, planning authorities, neighbours are managed promptly and professionally
  • Optimise orders, personnel on site, building regs visits and other arrangements for effective time management
  • Follow a clear decision making process to be on top of what’s happening on site

Digital platform

  • Take control of your project through houseUP dashboard, to monitor progress, budget consumption, see real time photos and videos, comment
  • Get involved in the process, the schedule, the choice of materials and finishes
  • Have access to a dedicated channel on houseUP chat platform, for fast trackable decision making and issue management
  • Keep a written record of all the conversations, questions, issues, decisions in one place, avoiding long email thread discussions

Understanding the Basement Conversion process

1. Free site visit

One of our construction specialists comes to meet you in person and view the site. We cover all areas in London.

  • Meeting in person: understand what you want and the challenges within
  • Seeing the place: place the vision in the physical constraints of your house
  • Understanding the context: collect details and impressions and explore the neighbourhood
  • Discussing your ideas: identify challenges, prevent issues, remove blockers

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 info 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, underground structures like road tunnels or tube tunnels. 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 a 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 a planning permission.

Maximising the value: understand the max you can achieve from your basement

Greenwich Three Storey House Basement Conversion

2. The preliminary process

In preparation for a basement project, you will need to take care of the formal procedures. They might include planning permission, building regulation, method statement, party wall agreements and so on. Our architects have experience in all councils in London

 (design and build only)

  • Understanding the context: evaluate all the physical factors that affect the conversion of a basement. Things to take into account are the load of the building above, the nature of the soil, the water table level, the underlying existing structures like services, tunnels, tube etc
  • Determining the formal process: understand the legal aspects and studying the council regulation; investigate the layout of the neighbourhood and the adjacent properties; define the need for party wall agreements, external technical reports, method statement etc.
  • Making the right decisions: maximise the value created by adding the basement, against the budget investment, when all the feasibility constraints are clear, choosing the best options for structure, design, usage, dimensions, layout etc.
  • Finalising the pre-construction phase: producing all the documentation including plans, technical drawings, structural calculations, method statement, work plan, submitting the plans to the council for permitted development or planning permission, serving the party wall notices and completing the party wall agreement

When converting a basement, the most important factor is to optimise the budget investment and maximise the generated value from your property. To achieve that, technical competence and experience are crucial to evaluate your options and make the right decisions.

Technical complexity, the neighbourhood, the length of the pre-construction process, the cost of external consultants and many more technical elements will support you for the best planning of your basement conversion

Maximising the value: how achieve the maximum for your budget investment

Basement Listed Building Rear View-min

3. Free construction estimation

One of our estimation specialists produces an accurate quotation, including a break down of all the details and their cost

  • Defining the costs: evaluate the basic construction work
  • Estimating the options: optimise the choice of finishes and suppliers for the best result
  • Explaining the quote: communicate clearly what is included or not and what might be the impact
  • Communicating effectively: explain the process and the program, define the communication channels, agree on the decision flow

Investing your money in renovating your property can be a big decision. That is why understanding the quotation is paramount to make the best decision. Most important of all, the price must be reasonable. Rogue companies often quote a very low price to get appointed, already planning to charge extras to make up for the difference; if not done on purpose, a contractor might choose to abandon the project when they realise it’s not profitable.
Check our infographic on 8 interesting facts that a good construction estimation should take into account.

Optimising the budget allocation: maximise the value of your property and the quality of your life.

5 Bedroom House Conversion Kitchen Pendants

4. Build

Basements need special attention to the construction procedure, because of the higher risk related to the existing buildings during the work.

  • Enforcing the method statement: create instructions for the team to carry out the work. Those include accurate descriptions and precise details, like props for vertical and lateral support, temporary roof, usage of conveyor belts for soil removal etc
  • Appointing the teams: choose experienced and well tested basement specialists to carry out the work and the best project manager
  • Supervising the site: stay involved with day-to-day activities on site, to provide guidance and coordinate the personnel, the supplies and the procedures on-site
  • Reporting and sharing info: give regular feedback and respond promptly to questions and requests
  • Setting up communication channels: dashboard, live chat, photo and video sharing using the latest tech solutions
  • Making timely decisions: follow a clear process for requests, approvals, last minute changes and all decisions

Building a basement requires technical competence and experience and basement contractors work better under the supervision of professional experts. On top of that, information sharing and effective communication are also important to ensure the best results.
That is why with houseUP you will have a structural engineer in charge of your basement conversion, plus dedicated communication channels to ensure the decisions are made promptly and the crucial information is always at hand.

Producing the best results: generate the maximum value for your money and your requirements through a hassle- and stress-free process.


5. Fit-out, completion and handover

Your construction project manager ensures you make the best decisions for your fit-out and finishes and the best tradespeople are appointed to deliver the best results for your basement

  • Finalising the approval of the finishes: although 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, fittings are all details that make a difference in the quality of a basement
  • Appointing the best teams: choose the right tradespeople for all the work on site; electrical and plumbing, for example, are crucial to a successful project and the wrong choice can cause a lot of problems in the future. houseUP ensures you get the best specialists for electrics, plumbing, tiling, painting, flooring, decoration etc 
  • Wrapping up all the final bits: make sure all the little details that need to be retouched are taken care of before hand over

While building the shell of your basement is very important, you want to make sure that the space you create is pleasant to be used and lived in. In that, the finishes are essential, and we will make sure you are involved and informed so that you can choose the best for your requirements and have everything executed at top-notch quality.

Managing the project to completion: take care of all the small details for a successful hand-over.


Tanking and Waterproofing

Tanking is a building technique for underground structures that controls the water that comes from the external soil, using draining channels to remove the excess of water, while the basement is protected from infiltrations by a tank that surrounds the walls and the base of the basement. For the tank, cavity drain membranes, channel drains and sump pumps are employed. The pumps are installed with redundancy and alarms. So 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. In the case of the basement, waterproofing membranes are added to the tanking, to prevent water penetrating the basement.

In houseUP we have the in-house expertise to design and implement the latest technologies and the most effective waterproofing systems, to ensure that your basement is built to last longer.


Planning and permitted development requirements

For basements in London, permitted development is restricted to not substantial alterations. You can convert an existing cellar or basement into a living space within permitted development rights.

Any excavation for a basement requires planning permission. Approval rate for basement conversions planning applications is very high, because councils usually grant permission. The process for the application, though, is a bit more complex, due to the amount of required documentation, reposts, investigations and other conditions that are specific to basement applications.

The final result is very predictable, but the process to get there is usually longer and has higher cost than applications for standard extensions  like loft conversions or rear extensions.

Basement conversion budget

A basement in London is relatively more expensive compared to other types of extension alterations, so it is important to understand the costs and the value generated. The value will depend on the existing property, the area of London it is located, the size and the quality.

Variables that affect the cost can be:

  • Structural design: an efficient structural design can save on materials and costs, with the same final result
  • Construction process: following the correct procedure and building in the right time of the year can avoid contingency costs
  • Soil constraints: the nature of the soil can require specific measures, like piling foundations or thicker retaining walls and slab
  • Party wall agreements: depending on the number of adjoining properties and involved parties, party wall agreement can add up to the costs
  • Design choices: using materials like structural glass, expensive internal finishes and other specifications can change significantly the final cost

Average cost

It can help to have an idea of how much your basement conversion costs as an average. It’s important to always keep in mind that many variables can add to the average cost; other conditions can make the cost lower. Make sure you work with the right company that can support you making the choices that are best for you, also considering your budget requirements.

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

Our basement conversion projects

Kitchen in a basement in listed building, London

For this basement conversion, we operated on a listed building. We converted the existing space into a two bedroom flat with an independent entrance at the front and a back door to the rear garden.

The building above has 3 storeys and is located within a Georgian block listed as Grade II.

Rear garden entrance of a basement in London

For this basement we also refurbished the external area, restoring it to its original paving with yellow Yorkstones

Open plan dining and living areas in a basement

For this basement we installed Italian porcelain tiles, together carpets and vinyl flooring

Dining area with front facing window, basement conversion, London

Our basements are always full of air and light. Here the good design for the ideal start for us to implement a pleasant and enjoyable new space for a growing family.



Gants Hill

We have recently moved back into our house after completion and could not be more happy.



They finished in time, were always available to give ideas and suggestions and most importantly, were very reliable, such a rare find in the construction industry!



They are truly a construction team you can fully trust and are highly recommended.

For more info on your basement conversion in London you can check our recommendations here.

Contact us to discuss your basement conversion and its costs. We’ll help generate the most value from your property, within your budget constraints, taking care of the planning, design and construction of your basement in existing and new buildings everywhere across London

Use our live chat, our email or our free phone number for more details, to book a free site visit or to get a free quotation.