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Basement Conversions

Basement conversions have become a significant trend in the last decade, as we look to add more space to our homes in order to stay living in areas that we love.

Whilst basement conversions do cost a significant amount of money, they can also offer a high return on investment. This is particularly true for London basements where the value of real estate per square metre is high. For example, if your property is worth >£7,500 per square metre, then you can expect a return on investment of >1.3:1

Types of basement conversions dependent upon your plot:

1. Under house basement conversion

This forms the majority, particularly for London basements where gardens are small.

2. Under front garden / under driveway basement conversion

These have become particularly popular with those looking to house their valuable cars in protected under-ground car parking spaces.

3. Under garden basement conversion

Ensoul are renowned for designing under garden basements. To go down this route, you will need a sizeable garden either to the front or rear of your property.

Generally speaking, planning authorities restrict under garden basements to 50% of your garden. It’s important to keep that in mind when thinking about whether it’s a viable option for your accommodation.

Under garden basements have the added benefits of:

  • Potentially allowing more light into your basement with glass roof lights, light-wells, bay windows etc.
  • Often, we are able to create a separate entrance for under garden basements which double-up as fire escapes.

Basement Services We Provide

Feasibility studies:

We will work through what size and broadly where your basement could be situated with consideration to:

  • Planning rules in your local authority.
  • Site and arboricultural restrictions.
  • Site drainage.
  • Party wall implications.

Architectural design and space planning:

This will provide you with plans and lay-outs of your newly proposed basement. You can then see the size and shape of your basement, how it connects to your current home and how the space will be divided and used. We also indicate key fixtures, fittings and pieces of furniture so that you can see how we would suggest fitting it out and furnishing it.

Submitting and managing your planning application with your local planning office:

We will create all of the plans and documentation required and submit to your local planning office. We liaise with the allocated planning officer, answer questions and negotiate where required. The process typically takes 8 weeks from receipt of your application.

Independent basement cost planning and procurement:

We create a detailed tender package and engage with 2-3 specialist basement contractors that we know and trust to build such complex structures.

We frequently break basement contracts into two parts for your basement build as each requires very different skills. It also means that you can get Part 1 underway for a fixed price, whilst we design the interior of your basement which is where the many cost variations are.

Part 1: Basement Shell & Core construction. This is the creation of your waterproofed, concrete structure in the ground.

Part 2: Basement Fit-Out. This element is focused around preparing and fitting-out the interiors:

  • internal wall formation
  • plastering
  • flooring
  • electrics and plumbing
  • lighting
  • decoration
  • bespoke joinery, etc.

Why Ensoul instead of going straight to a basement builder?

Best design and space plan: As architectural and Interior designers, we understand how to bring in the maximum light and best utilise the internal space. Builders will often design boxes with small, uninspired rooms for ease of construction and maximising profits.

Planning success: We understand the planning rules and have a high success rate with getting plans approved.

Best value: We know how to structure and negotiate the tender process with building contractors and specialist third party surveyors. We can compare these against our own internal benchmarks to get best value for you.

Rigorous waterproofing design to BS 8102:2009: Our design team includes specialist water-proofing designers who get involved at the very early stages of the technical design process. This is before any building contractor starts on-site thus ensuring waterproofing is designed-in as opposed to an after-thought.

Award-winning interiors: We design highly creative spaces with wow factor.

Peace of mind: We will check that you are signing up to building contractors with the appropriate guarantees and insurances.

On-time & on-budget: We offer experienced project managers that plan every step of your basement conversion. We keep your project on-track, on-budget and on quality.

Basement FAQs

How much do basements cost?

This is the question we get asked the most. There are typically three key cost buckets for building basements:

1.Professional fees: £60,000-£120,000+VAT.

This will vary depending on the scope of services and scale of your basement conversion and includes specialists such as: architectural designer, structural engineer, geotechnical surveyor, drains surveys, party wall surveyor, interior designer, project manager.

2.Basement shell and core construction: £2,700 to £3,700 +VAT per square metre. This is the dig-out and creation of your watertight concrete box. This includes: perimeter walls, underpinning, concrete floor slab, roof slab, staircases, waterproofing, ground and foul water drainage and architectural glazing. This should also include insurances and guarantees from the basement contractor.

3.Basement fit-out: £1,200 to £4,000 +VAT per square metre. This is for all the internal elements of your new basement e.g. flooring, electrics, lighting, AV, tiling, sanitary ware, built in storage etc.

For more information on basement costs please read our blog basement conversion costs

How long do basements take to build? Based on creating a new 100 sqm basement.

  • Basement architectural design for planning purposes: 6 weeks.
  • Planning application: 8 weeks with most planning authorities.
  • Structural engineering & technical design to build to: 6-12 weeks
  • Party Wall Award: 6-12 weeks
  • Watertight shell and core construction: 5-8 months depending on size
  • Fit-out: depends on volume and complexity but allow 2-4 months.
  • All in, we recommend that you allow approximately 15 months.

Can I live at home whilst my basement is built?

In most cases, the answer is yes, depending on the scale of basement and site access.

How big can I make my basement?

You can often go under the entire footprint of your house and up to 50% of your garden. Subject to local planning rules and your site.

Do I need planning permission for a basement?

Yes, in most cases you do need planning permission for a basement.

Do I need a party wall agreement?

Yes, in most cases. This is because a party wall agreement is required if you are constructing within 3 metres of your neighbour’s property.

Will my basement leak?

With the right team it won’t. However, basement waterproofing is complex and you need to make sure that you have experts designing it from the start and specialist builders.

What guarantees should I be looking for?

You should be looking for an Insurance Backed Guarantee for the “basement watertight shell and core”. This is an independent insurance policy that you can call on if your basement requires remedial works and the builder is no longer in business to fix it under their “10 year builder’s guarantee”.

Why create a basement extension?

  • If you need additional floor space and you have already extended as much as possible above ground. Basements add a significant amount of space to a property. You are often able to go under the entire footprint of your house and there may be opportunity to dig under your front and/or back garden.
  • You’re looking to add significant value to your property. Although basement conversions are expensive, if your property is worth >£7,500 per square metre, then you can expect a return on investment of >1.3:1.
  • Extending your property and staying there will provide you with an investment for the future and save you the money on moving costs.

What accommodation can I put in a new basement?

  • Cinema room, games room, kids playroom
  • Bar, dance floor, bowling alley
  • Nanny, granny, teenage flat
  • Gym, personal trainer space, yoga or Pilates studio
  • Swimming pool, sauna, steam room, spa, massage room
  • Wine cellar, wine tasting room and cigar room / walk-in humidor

If you would like to read more, look at our step by step guide to basement conversions


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