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Teams of Diamond Construction specialists use only the best materials, equipment, as well as technological and functional solutions to comprehensively and quickly conduct all the necessary work. This includes, for instance:

  • plumbing,
  • lighting,
  • damp proofing,
  • electrical work,
  • additional tasks.

We are the basement company, which focuses on the needs and expectations of every customer. That is why we offer coordination on each stage of the project realisation – starting with adjustments to the initial project, through all of the changes according to your vision, up to final improvements and even the tiniest corrections. Our teams have successfully conducted numerous basement extensions in London, which allowed them to gain experience in remodelling the space in traditional or modern style, implementation of innovative technological solutions, as well as transformation of space of very different types and sizes. What is more, the cost is always adjusted to the customer’s possibilities, without compromises in terms of quality of our work.

Infographic about basement conversion

Basement conversion infographic

Basement conversion cost

Basement extension is a great way to gain more functional space and get the maximum benefits from the extension process. At the same time, it is possible to increase the property value, typically by around 20% of its value.

Creating a space underneath your house can be a really cost-demanding process, especially if excavation and damp-proofing is required, as it is more expensive per square meter than a common above-ground extension.

There exist two main ways of using the space under your property and making it a habitable environment:

  • renovation of existing basement or cellar;
  • new basement construction underneath the house.

Each of these cases has its peculiarities which influence the costs significantly. Let’s consider all the factors in detail, so that you can avoid surprising and unpleasant situations once the process has already started.
Conversion of an existing cellar is not as expensive and may cost approx. £750 – £1,400 per square metre. It is necessary to take into consideration the features of the existing structure of the basement, the number of walls which will be built, individual installations and flooring design.

Factors that may increase costs include moving services, especially in the case of poor access to the site, underpinning of the existing structures if necessary, water-proofing, etc.
As for the new basement construction, costs may can vary significantly and usually include the following constituents (approximate costs per square metre):

  • Planning and Building regulations applications (£750 – £1200)
  • Party wall agreement (£700 per each neighbour)
  • Engineer fees (£1000 – £1500)
  • Floor level lowering and cellar underpinning(£1500 – £2000)
  • New basement space excavation (£1500 – £3000)
  • Lightwell and external access construction (£5000 – £7000)
  • VAT contractors additional fees may reach up to 20%.

To sum up, a shell place construction will cost £1,920-£2,640 per square metre. If you have an already existing cellar, converting it will cost £720-£1,800 per square metre.

All the above listed figures are approximations and may vary considerably depending on a number of factors. Moreover, they don’t take into account the design and interior or exterior finishes: wall painting, furniture, flooring, additional fittings or any other decorating ideas.

Diamond Construction Ltd always adjusts the costs to the customer’s budget capabilities, without lowering the standards and quality of our work.

Experts and Specialists

Our experts and specialists have years of experience designing and building basements. You might want to create a basement tailor-made to your tastes, just remember that you are not alone. Our designing experts will make sure you get the layout and design you want while pointing out useful additions and adornments that will make your basement cosy and modern, whether you want to convert your basement to a bedroom, a living room, or a home theatre.

We have expert architects, which have faced the unique challenges that come with renovating and overhauling basements. Basements are big spaces that don’t have much access to natural light, and without the advice and supervision of an architect, you’d end up with a basement that doesn’t have sufficient ventilation. You wouldn’t want a room where you and your family feel stifled and uncomfortable, and our basement experts are there to prevent it.


Trust the experience and professionalism of Diamond Constructions Ltd. andconvert, extend your house or get fantastic bespoke furniture for your interiors.

  • Financial Responsibility to Our Clients
  • On Time and on Budget
  • Real focus on Customer Satisfaction
  • Superior Quality and Craftsmanship
  • Quality and Value to the Project we deliver
Concreting the Basement Extension


  • What kind of costs would I be looking at?

  • Is planning permission obligatory?

    In most cases it is not needed, as the changes are conducted only on the inside of your building. However, in case of some projects it might become a necessary requirement. For instance, when the plan involves adding an entrance or windows on the front side of the property, digging completely new basement or digging up a pavement or part of the road outside your house, the local council must first provide you with a planning permission.

    When you think of extending your rooms or changing its structure in the future, it is advised as well to familiarize yourself with planning permission specifics for basement conversions. Different regulations apply to a listed building. Before starting any work it is necessary to acquire planning permission, also in the case of changes not visible from the outside.

    It is required because such properties are under protection for, for instance historical reasons. Remember, that even if you do not need a planning permission, the conducted work must always comply with building regulations. They are a set of regulations, which apply for all new living space and guarantee the residents’ safety.

  • Do I get any financial benefits?

  • Can my basement be converted?

  • Is there something else I need to consider?

Basements and Party Wall Notices

A Party Wall agreement is a very important document which must be signed before construction commences. It is a legal agreement with your neighbours to begin works which may affect the jointly owned boundary between the properties.

Party Wall agreement is a very important step in the process of planning your basement conversion. It concerns the owners of the property where the conversion is going to be made and all neighbours who live next door to the renovating property.

If the neighbour does not sign the agreement, then the work will be blocked and cannot be started. After receiving the notice the neighbour can choose to consent to it or not. Consent completes the process, dissent puts the parties in “dispute” under the “Act”. The dispute is resolved by appointed party wall surveyor who produces an agreement and resolves the dispute. That agreement is known as a party wall award. If the neighbour does not give their response before the notice period expires, the owner of the property can commence with the work. The surveyors can become involved again if new disputes arise during the extension works.

Related projects of basement conversion

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Diamond Constructions is about far more than just refurbishment services.

In order to bring you the best quality services for your home, we’ve expanded our range to encompass kitchen joinery.


Visit our new website – Diamond Kitchens


  See for yourself just how much easier we can make decorating your home.

Once there, you’ll find a host of interesting articles pertaining to kitchens, a portfolio of our kitchen projects, and many other useful tips to go along with our impeccable level of service translated into kitchen design.