Trimble Architects

RIBA Architectural Services

Phone Number: 020 8538 9303

Mobile Number: 07785711635

 Are you looking for basement conversions architects in London? Our RIBA local architect services works throughout Hounslow, Twickenham, Richmond, Kingston Upon Thames, Teddington And Throughout London.

Basement Conversion Services

Suppose you wish to use the space within your London household by transforming it into a specific purpose or enlarging another room in your living area with a high-quality basement conversion. In doing so, you can create a unique space that is both unique and practical. 

For those located in areas that make traditional house extensions in London unfeasible, you can successfully convert your basement to create the ideal space your after to host guests, relax or store your belongings.

There are so many possibilities you may choose from to curate out of a basement conversion, including playrooms, home offices, extra bedrooms, home gyms, kitchens, bathrooms, etc. 

One of the most significant advantages of a London basement conversion is adding additional living space inside your house without losing outdoor space or going through a completely new building project where planning approval is most challenging to secure.

A London-based basement refurbishment can dramatically increase your existing property's value, especially in royal boroughs and more non-rural areas, where extra space is considered a luxury. 

Why go for a basement conversion?

Basement development is a fantastic option for those who desire to add more space to their households and properties.

Suppose you cannot extend outwards or upwards due to strict building regulations on the style of home you obtain. In that case, you can opt for a basement conversion, providing you with a similar amount of living space without the need to obstruct the external property.

In London, space is an incredibly valuable asset to purchase at a cost-effective price. When converting the basement, you can achieve the extra room you desire that could change your life and enhance your family's life.

Materials and technology have advanced over the years, allowing basements to become much brighter areas of the household and no longer reduced to dark, scary, low-ceilinged cellars. When relying on trustworthy professionals, you never have to worry about damp-proofing or waterproofing your basements.

If they have been skillfully designed by a qualified, creative professional, such as ourselves, you can look forward to a basement that offers any unique purpose within the home.

The road to basement conversions in London with our company is hassle-free and a breeze from the first free consultation to the planning stage and through construction to the final installed result.

We're responsible for ensuring a structural engineer is in charge of the basement conversion process for maximum security and safety on-site. 

 Basement Conversions Architects London

How much does a basement conversion cost?

You can expect to pay approximately £140,000 (+VAT) and upwards on an average basement conversion; however, consider various factors determining the cost of basement conversions.

For example, costs will factor in size, materials for flooring and the foundations, plumbing, location and whether or not the digging will require engineering fees. 

Once your basement conversion has been extended outwards or upwards in your backyard or garden, it can add plenty of necessary extra space to your London home and enhance its value. An average basement conversion may cost from £140K upwards.

Yet, numerous aspects are included that impact the final price, for example, the additional fixtures required and the size of said fixtures. You can utilise a brand-new or extended basement for many purposes, such as studios, swimming pools, extra bedrooms for growing families, wine cellar conversions or purely for more storage space; the list is virtually endless.

Basement excavation projects can often become cheaper, including waterproofing, underpinning and tanking, alongside many others, yet this generally depends on the area you have and its size. It's always best to inquire before beginning the planning process as conversions in London basements will always vary concerning the cost of converting.

Don't hesitate to contact Trimble Architects Ltd today; our professionals can discuss a wide range of basement conversion requirements with you and offer all the support you need. We'll book you in for a free no-obligation site survey so that our planning professionals can assess the property site to garner the necessary structural calculations.

Call our phone number or send a message to our email address to receive a free quote or site survey, acting as dedicated project managers. Our staff will inform you upon our initial consultation about our comprehensive architectural services that you can utilise throughout your basement project. 


If you wish to live in the centre of London, you'll find that a lack of space may be something you'll have to live with unless, on the other hand, you have the budget to undertake total London loft conversions double-storey or single-storey extensions.

Another effective and cost-effective way to maximise your space is to have more room for storage without obstructing the outdoor area of your property.

Whilst a dark, cold space without any natural light can sound quite unappealing, many potential buyers are intrigued by the space's versatility for their belongings.

And, of course, it doesn't have to be without natural light, as builders can add light wells that penetrate the roof level onto lower levels.

As your basement will then be separate from other places in your household, you can thus create an interior that is practical and suits your needs without worrying that it needs to fit the themes of the rest of your home.

You can rest assured that your brand-new converted London basement will be successfully cooled and heated, much like the rest of your household. 


London basement conversions are gradually becoming some of the most popular forms of home extension and home improvements, as it provides boundless possibilities. 

Suppose you don't have an existing basement in your house or property; our fantastic London architects are fully qualified and have numerous capabilities and extensive knowledge in excavating downwards to bring your vision alive.

After working on a whole host of projects on a wide array of properties with varied end goals, our team of architects and construction team pride ourselves on our ability to tailor our building work and design process to suit each client.  

We'll ensure you get the best out of your household and our company's services. We work alongside some of the best structural engineers or extension builders from our sister company to get the most accurate and innovative build. 

Frequently Asked Questions about Basement Extensions

Various factors impact the cost of your basement extensions, including its size, basement design, building type, location and geology and the specifics of the area or existing building, for example, whether it is a listed building or in a conservation area. 

The Basement Waterproofing Association has since provided evidence of the most recent costs involved that you must consider when extending your basement:

  • Lowering your basements floor level will cost around £1,500-£1,200 per square metre.
  • Digging a brand-new basement space and underpinning costs between £2,500-£3,000 per sqm.
  • Where necessary, planning application council fees are included, which may cost around £206 for a householder application and £462 for the proposition of a brand-new accommodation.
  • A building regulations application will cost around £250-£750
  • A party wall agreement, if necessary, will cost anywhere from £90-£700 from a neighbour.
  • Digging a brand-new basement will cost approximately £1,500-£2,000 per sqm.
  • It will range from £900-£1,400 per square metre to convert an existing basement.
  • When creating external access or light well, the price tends to sit around £5,000-£7,500 per sqm.
  • The additional tax will also be +20% for those utilising registered contractors. 

Suppose you have plans to convert your current basement without needing to adapt the property's external appearance or excavate the building. In this case, you typically won't require planning permission to complete the extension project, as it will be regarded under permitted development.

However, you will likely require planning permission if planning on a complete retrofit of their existing basement through adding ventilation, new light fixtures or additional significant structural changes and elements, as this is a choice that alters the structural integrity of the building.

It is the same for those wishing to create an entire new basement from scratch; however, you may also need Supplementary Planning Documents (SPD). It's vital to check by contacting your local planning authority and inquire about what must be carried out for you to gain the necessary planning permission, as several kinds of assessments and surveys determine the feasibility of your desired basement construction.

We recommend researching to be as prepared as possible to make any essential changes to comply with the building regulations and potential party wall agreements. 

Basement extensions can thoroughly increase your household value by approximately 30%. For those living in areas or boroughs around London where extra property and storage space is rare, such as Hammersmith and Fulham, one of the more popular extension types is basement conversion. However, there are various aspects that you must assess to gain an indication of the final cost of your cellar conversion.

Basement additions tend to add plenty of property value, especially for potential buyers who need more space for storage or central heating appliances. To investigate the potential cost, you'll require the assistance of an architect or building surveyor to help you decide what factors increase the overall cost of your household. 

Trimble Architects

Based in Twickenham

South West London

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If you require help and advice from our basement conversions architects get in touch today. Trimble Architects are based in Twickenham and work across south-west London.

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