We've created a step-by-step guide for those who are thinking about extending their house with a basement extension or conducting a basement conversion in London

For those considering a basement under their house in London, we recommend you dig deep (4M-4.5M) to get a high ceiling (2.7M to 3M), put in light wells (and roof lights where possible) to get an incredible space that feels just as good, if not better than your accommodation upstairs.

These are some of the rooms and types of accommodation that we see basement conversions result in:

  • cinema room
  • kids playroom
  • games room
  • wine cellar and wine tasting room
  • cigar room / walk-in humidor
  • gym
  • sauna
  • swimming pool
  • spa
  • massage room
  • sanctuary
  • nanny, granny and teenage flats
  • bar
  • nightclub dance floor
  • bowling alley
  • recording studio
  • luxury car park


This guide will explain how Ensoul Architectural Design can take you all the way from initial interest, through the planning application process and then into construction and handover of a basement.

London Basements can add significant value to your property; for example, if your property is worth >£750 per Sqr Ft, then you can expect a return on investment of >1.3.


child playroom basement conversion
London basement converted to playroom

Step by step guide to basement conversion London

What follows is a comprehensive, step-by-step guide to London basements from first concept through to final handover.

It will include informative policies, space planning and recommended professional services based on previous basement conversions. This same sequence equally applies to under-house as well as under-garden basements although some of the emphasis/detail will be different in some of the steps.


Step 1: Understanding the planning rules

Planning restrictions

Every planning office has a different policy regarding what you can and can’t do with basements conversion in London, varying from each borough. It’s therefore important that you investigate the latest basement policy and guidance of your local authority at the very start so you know what kind of basement is feasible, the requirements attached to the application and any design or cost implications. For example:

  • Wandsworth basement policy states that when considering a lightwell in the front garden “you must ensure that at least 50% of the front garden remains following excavation of a “lightwell”.
  • Merton basement policy stipulates not to exceed 50% of either the front, rear or side of the garden of the property for constructing a basement.
  • Royal Borough of Kensington & Chelsea basement policy limits the extent of basement excavation to no more than the depth of a single storey.
  •  And Westminster’s new basement policy has introduced a levy from 1 September 2016 for a subterranean squad of “basement nuisance busters’ – charged with overseeing and controlling traffic impact, stricter working hours and complaints. It’s the first of its kind in the country and assumed to be picked up by other boroughs eventually.


Conservation area restrictions

If this is applicable, you will need to ascertain any additional restrictions placed on a basement conversion in your area.


Step 2: Ensuring Value

You need to conduct desk research and talk with local estate agents to establish what your basement will add in terms of additional £/SqrM. You can then ensure that the basement cost in London versus value created is in balance and that you get a great return on investment.

We have seen the ratio of “incremental property value per square metre” compared to “build cost per square metre” as high as 2:1 in some cases.


Step 3: Surveys

Before you start to think about the design and planning application, you will need to get surveys done to inform the basement technical design and possibly for inclusion with your planning application:

  • Geo-technical survey – this reports on the soil structure down to the depth you are digging and any likely issues regarding the height of the water table.
  • Drains’ survey to ascertain where the drains run around/under your house and garden to understand the impact of the basement construction. Depending on where the drains run, a separate “build-over” agreement may be required from your water utility provider.
  • Topographical survey to get accurate dimensions of the site.
  • Arboricultural survey to investigate trees in yours and your neighbours’ gardens to see what impact a basement dig may have on the health of any trees. Also, as part of this step, identify any that fall under a Tree Preservation Order.
  • Flood risk assessment where applicable


Step 4: Architectural design & space planning

Once armed with local authority policy, the necessary survey results and an outline budget estimate, you will need to design your basement space plan which will include:

  • How big and what shape it will be
  • How you propose to connect your new basement to your existing house
  • What’s the best space plan and room lay-out
  • A high level interior design plan and furniture lay- out so that you know the architectural design is in perfect harmony with the interior plan
  • Where to put services i.e. drains, electric runs, ventilation, etc.
  • Access and exit points including compliance with building regulations
  • Roof lights and light wells to provide lots of natural light and ensure a high quality living space


Basement conversion to play room
kids’ playroom in basement – London

Step 5: Neighbour Support

At this stage and definitely pre-planning application, we recommend making the time to book 1:1 meetings with your close neighbours to talk about your plans over a coffee or a glass of wine. Ensoul can help to create a presentation pack for you and can attend if it’s helpful to answer technical and build questions.

The main things to communicate are:

  • Explain why you’re wanting to create your new space and what it will mean to you / your family. Neighbours are far more supportive if they understand that you need more space for music practice with contained noise levels, or space for a live-in nanny so that you can go back to work, etc.
  • Provide them with a top-level overview of the proposed basement size and design.
  • Go through timings with them so they can understand the duration of works and impact to them.
  • Explain that it is being professionally designed, built and managed to quash any fears about “cowboys” potentially collapsing your houses.
  • Make a small gesture to them e.g. pay for a window cleaner once a month during works. The goodwill will go a long way and in the scheme of things, the cost is minimal.
  • Critically, get any concerns or questions answered there and then, or written down so that they are dealt with immediately or if not there and then, before planning. This will prevent potential complaints or open comments on your local planning website when you file your application.

Step 6: Planning Application

Next, finalise your plans, write the necessary content and file your planning application which will take up to 8 weeks from acknowledged receipt by your local planning office.

Typically, your assigned planning officer will want to visit and will have some questions that need answering and are likely to indicate some conditions that may be required to give planning consent. Ensoul can handle all of this on your behalf through to conclusion and result.


Step 7: Technical and structural design

Structural design and calculations:

A structural engineer now needs to be appointed to decide and detail the best way to design, support and build your basement shell and core. The ‘shell and core’ is fundamentally the reinforced concrete structure in the ground. The engineer’s design will factor in all survey information available as well as your project timescales and budget constraints. The resulting piece of work is a series of drawings and calculations which identify the construction sequencing, materials, steelwork, under-pinning and/or piling details, party wall detailing and CDM (Construction Design & Management) requirements. The structural design is required before the Party Wall Agreement process starts as any appointed surveyor will want to review these as part of your Award.


Waterproofing design:

How the basement structure is designed will also determine the best waterproofing system design. On the assumption that your basement is structurally sound, waterproofing it is the next most important aspect of your build. A leaking basement creates enormous stress and is an incredibly difficult problem to solve. Because water travels, it can be very difficult and in some cases, impossible to find the entry point. So, you must ensure you have the very best waterproofing experts designing your basement from the start. The way the waterproofing system is designed also has implications for the insurances/guarantees you will need to put in place.

Some of the larger specialist basement construction companies have extensive skills in this domain in-house. However, it is a highly specialist and skilled area and when using smaller firms, we recommend engaging an experienced waterproofing design consultant to work as part of the design team. If you would like to find one, either talk to Ensoul or search BSWA .

Key areas of consideration in your waterproofing design include:

  • management of surface water / land drainage
  • sump pumps and their discharge paths
  • anti-lime treatments
  • treatment of day joints and use of water bars
  • post construction maintenance and flushing points
  • hydrostatic pressure implications
  • flood testing post construction

Party well agreement:

If your planned basement is within 3 metres of a neighbour / boundary wall then a party wall agreement will be required (there are exceptions where deep foundations exist).

The required notice period prior to work commencing is 8 weeks and typically takes 4-8 weeks to negotiate (from the time notice is served) for relatively straight forward projects; in more complex cases they can take significantly longer.

When initiating party wall agreements, talk to your neighbours and see if they are happy for one Party Wall Surveyor to be appointed to represent all parties. According to the Party Wall Act 1996, they are obligated to remain impartial and thereby protect everyone’s individual interests. Neighbouring owners may however prefer to have their own surveyor appointed which means it can take a bit more time, and expertise, co-ordinating them all.

If you are looking to engage a party wall surveyor, either talk to Ensoul or go to: FPWS   and/or RICS


Step 8: Engaging a basement contractor to build your basement

We recommend that you think about your basement works as two distinct projects:

Basement shell and core:  this is the creation of your waterproofed, concrete structure in the ground. A specialist contractor, as opposed to a general building contractor, is required for this basement construction stage. Make sure that you are contracting the actual contractor that is going to conduct these works – not a firm that is going to subcontract these specialist works and simply put margin on it. .

Basement fit-out:  this element is focused around the interior finishes and includes for example, wall formation, plastering, flooring, lighting and electrics, plumbing, decoration, joinery, etc. Most basement contractors only focus on delivering shell and core packages and are not interested, or sufficiently experienced, in delivering fit-out work. There are some that can do both for you.
The most efficient way of constructing your basement is to get your Shell and Core contracted first as a detailed package of works with a fixed price. This way, you can get your project up and running and whilst your basement is being dug out, you can be designing the interior of your basement (either yourself or with a professional), and selecting all the internal finishes. Once this work is done, you can get an accurate scope and budget put together by your design firm and/or fit-out contractors.

Contracting the shell and core:

Once in receipt of the approved architectural plans and structural engineer’s drawings and calculations, you will need a detailed scope of works ready to discuss with pre-vetted basement contractors.

At this stage, either:

  • a tender is written and distributed to compare market prices and approaches, or,
  • a “competitive commercial dialogue” process is used to get to a preferred supplier quickly.

Producing a short list of competent contractors to discuss your basement project with requires expert knowledge and references. As the trend for basements increases, more and more builders are bidding for business. Very few, however, have the technical know-how, experience, quality of people and credentials to do the work and provide the required guarantees so due diligence is essential. It’s also essential to take out client references from people who have had their basement done several years before, so that you can ensure they’ve had no problems over a sensible period.

Once a decision has been made on which basement contractor to appoint and there is an agreed fixed price quotation, there are several contracting options. We advise our clients on a case by case basis as to which one is the most appropriate route for them,
for example:

  • JCT Design and Build Contract (watertight shell and core)

This provides a single point of accountability for all elements of your basement shell and core design and build including:

  1. Structural and temporary works design
  2. Waterproofing design and drainage
  3. Basement shell and core build
  4. External glazing
  • JCT Intermediate with CDP (Contractors Design Portion) and minor works contracts

These contracts are typically used when it makes sense to break out one or more elements of the basement design and/or build to specialist 3rd parties. This decision may be taken if the preferred basement contractor has a lack of expertise / experience with a particular element, or because there is a significant cost variation between having it all under one roof vs taking some elements out of scope.

For instance:

  1. The structural design could be contracted with an independent structural engineer
  2. The waterproofing installation could be contracted with a specialist waterproofing professional.

Guarantees and insurances:

We strongly advise that you take expert advice on guarantees and insurances.

If the unthinkable happens, your basement leaks, and your contactor has gone bust, what happens next? Who are you going to try and claim against? Or, if you decide to sell the property and the buyer asks what guarantees are in place, how are you going to give their conveyancing solicitors comfort that all the guarantees are robust.

The fundamental elements to consider include:

Builder company guarantees

  • Waterproofing guarantees (design, product and installer)
  • Insurance backed guarantees
  • Latent defects insurance
  • Building’s renovations insurance (Joint Names)


During the build, an approved Building Control Inspector will need to be appointed to oversee the construction work and sign off that the structure conforms to Building Regulations. This will require visits at key stages which will be co-ordinated by your basement contractor.


Excavation and building the concrete box

Now the works can start and the next level of complications begin which is managing the basement contractor on-site to deliver against the design details that have been developed without variation. This deserves an article in its own right rather than describing the extensive detail here.


Step 9: Interior Design

If you have not done your interior design beforehand, you can do so once your basement contractor starts works on the shell and core as this element will take a few months before completion.

Your basement interior will need designing and all fixtures and fittings will need specifying including:

  • roof lights and light wells
  • windows, external and internal doors
  • staircase, floor and wall finishes
  • sanitary ware, tiles, paint colours
  • AV, security and IT networks
  • HVAC (heating, ventilation, air conditioning)
  • light fittings, furniture selections

For a beautiful result and efficient flow of works, hire an Architectural/Interior design company to put together a cohesive scheme and to recommend full product selections to you. You would be amazed at how many products and finishes you will need to pick and agree so we recommend you bring in professional help.

If you’re bringing in a designer, make sure they can handle the technical design such as lighting plans, floor make-ups, glazing details, etc. as well as the ‘pretty stuff’. It also pays to ensure that your design team can deliver a technical CAD package of drawings and detailed schedules so that you can see exactly what you are getting and sign it off; and so that the builder knows exactly what to cost and then build to with no ambiguity.


Step 10: Fit Out Contract

Once you have your agreed fit-out package of drawings and schedules, it can be sent out to fit-out contractors to provide a fixed price quote. You or your designer will also need to get all of the products quoted as well by all the various suppliers once you once you have chosen everything to create the fixed price budget.

By selecting everything before the fit-out starts, you can make sure that your basement interior design is well thought through and cohesive and that you are getting an accurate fixed price quote from builders with everything detailed and included.

It also means that you can fix a basement cost at the start with a contractor and ensure that when they start works, they have all the information they need to deliver the programme of works.

under garden basement extension conversion gym

London basement converted to gym 


Article summary and conclusions

There is a lot of detail to be managed, third party service providers to co-ordinate and tendering processes to go through. You should consider getting an independent Project Manager with experience in basement construction to manage the process from design to completion. We’ve also written an article on the benefits of professional project management and defined the key areas of responsibility / scope of works that one should be contracted to deliver against Professional construction project management

One of the common questions we get asked is “How long will my basement conversion take to build once contractors start work?”. For an average basement of 70 sqm and one storey deep, allow approximately 9-11 months which breaks down broadly as follows:

Watertight shell and core 5-7 months:

  1. Site set up
  2. Excavation, temporary works, dewatering, underpinning/piling
  3. Floor slab and sump pumps
  4. Staircase and roof slab
  5. Waterproofing and drainage
  6. Underfloor heating and screeding
  7. Rooflights and lightwell glazing
  8. Testing & certification
  9. Site demobilisation

Fit-out: depends on complexity of build/finishes but allow 3-4 months

Landscaping (if it affects your garden): depends on complexity of design but allow 1 month (this can occur in parallel with the interior fit-out)

Furniture & soft furnishings: subject to order dates and lead times

In conclusion, basements add a fantastic amount of living space, can significantly improve your quality of life and add material value to your home. Our advice to anyone embarking on basement conversion in London is to:

  • Engage an experienced design team at the concept stage to ensure you optimise the space design, room lay-outs and quality of light to deliver a well designed, high quality living space.
  • Ensure that you bring in experienced and capable contractors who know how to safely construct basements and make sure they’re watertight. Calculate the basement cost but beware of going cheap.
  • Bring in expert knowledge to write and package up your contracts with the builders and to make sure that you have the right insurances, guarantees and warranties in place.
  • Consider employing a professional project manager, experienced in basement builds, to manage contractors and third party professional firms from design right through to hand-over.

Want to find out more or get started?

Call us today on 020 3637 0700 or email transform@ensoul.co.uk 

Download the PDF

Download or read The London Guide to Residential Basement Extensions and learn how we’ll work with you from concept to planning approval.