OLBC Design & Planning

Basement Conversions – design, planning and construction questions

My existing cellar / basement has limited headroom and feels cramped. Will my new basement conversion make a real difference to the feel of the space?

We typically design a basement to have at least 2.7 metres of finished headroom, more than your average typical design and build company would typically propose. We feel it is important to preserve the perspective of room size and height from the existing floors above so the new space doesn’t appear confined in comparison. If you have a period house, it is also likely you will have a step down to the rear part of the ground floor. We will ensure this step does not continue in the basement floor, giving you more flexibility with room layout and providing even more headroom at the front of the property.

I’ve seen other basement conversions that feel a bit damp and dingy.
How will mine be different?

Waterproofing technology has come on leaps and bounds since the early cementious tanking systems were used to ‘block out’ the natural passage of water. Most modern conversions use a plastic cavity membrane system that allows the moisture to run uninterrupted behind the membrane into a sump beneath the floor, which is then pumped away into the drainage system. This means that no differential pressure is created either in the structure or across the membrane, and also that a vapour barrier as well as a water barrier is created between the earth and the structure.

Do I need to get plans drawn up before beginning my basement conversion?

Yes, you will. For large structural projects detailed plans and specifications are a must as these enable us to provide accurate timescales and costs. They are also required for planning permission and for the engineer to work from. We can undertake this for you as part of our preworks service. Please call us to discuss.

How much does a basement conversion cost in London?

Fortunately in most cases much less than the resale value of the new space! Prime quality basement space done correctly can be worth as much as any other space in your home, so the value goes up in line with the increased floorspace. However, poorly thought out space can significantly reduce the potential value gain. Time and time again agents (and happy clients) have told us that our basements are so well thought out and gracefully conceived that the value that they add is massively over and above the relatively minor extra costs associated with elevating the standards. Please call us and we will propose the best way to give you a realistic price for your project.

How do I find out what permissions I might need?

Planning permission is usually required for large structural alterations or additions to existing buildings, and the erection of new buildings. Planning permission is not required for all types of works, as some basement conversions are classed as ‘permitted development’. We will look after these issues and obtain all relevant permissions on your behalf – please contact us for more information.

Listed Building Consent: If your house is more than 170 years old, or of special architectural or historical significance, it may be Listed which means you will also require Listed Building Consent in order to carry out works. Such buildings are described as being either Grade I, Grade II* or Grade II in significance. This Listing potentially covers both the external and internal aspects of the structure. We will be able to advise you and seek the relevant permissions on your behalf. OLBC has expertise in these areas and we are a particularly good choice for your project if you have any specific concerns with regard to the safeguarding of any material of your property, or require any advice on sensitive restoration or conservation work. Read more about our listed build services here.

Do I need engineering for my basement conversion and why can OLBC provide the optimal solution?

It is absolutely essential that the structural design and calculation for your basement conversion is carried out by structural engineer highly experienced in basement design. Fortunately, OLBC works with the most experienced basement engineers and so can form the optimal team.

Do I need party wall awards for my basement conversion?

Party Wall Awards are usually required for major works to terraced or semi-detached houses and sometimes for detached houses too. You are required to arrange for a Party Wall Surveyor to agree an Award for the work before work begins on site. A Party Wall Surveyor is an independent professional who will draw up a condition report detailing the structure and condition of all properties concerned, and will include plans and engineering for the proposed project, and details of any specific clauses or precautions which must be taken during the work. Your neighbour is entitled to allow your Party Wall Surveyor to act on their behalf, or they may appoint their own Surveyor, a cost which would be passed on to you, as the proposer of the work. Party Wall Awards may not appear on the surface to be a large concern, but they can have a great impact on the time it takes to get a project onsite. We strongly recommend you contact us before appointing a surveyor so we may help you to minimise the problems that can occur. OLBC can recommend some of the most experienced Party Wall surveyors in London.

Should I hire an architect or engineer for my basement conversion and how do I find one?

If you have not already commissioned your design and planning or engineering, we would encourage you to come direct to us for the entire package. Not only can we offer you a cost-effective end-to-end service, we can free you of the responsibility of introducing professionals who may not have worked together before and may not share the same working approach. With our complete service, the responsibility for delivering your project to site rests with our professionals who are experienced at working together to arrange basement excavation projects.

When can we start and how long will it take to complete my basement conversion?

OLBC provides an accelerated time path to get your project onsite. By removing the iterations required between professionals we can offer a tight timetable. The preworks element which is necessary to complete before work begins onsite will generally take from 3 to 4 months. In this time, we need to survey the site, agree plans for the basis of the new basement space, create a full quotation and specification, acquire planning permission, engineering, apply for street licenses and notify Building Control.

When on site the size, specification and ground conditions of the basement determine the duration of the excavation. OLBC will be pleased to provide a skeleton program to clients. You will be kept fully informed both before and during the project as to the timescale.

Will we need to move out during the works? If we stay, will there be much disruption?

If you are just excavating a basement in a house with suspended timber floors in most cases it is feasible to live in the house. If we are undertaking works beyond the basement, we usually recommend that the client vacate the premises. This has a positive effect on coat and duration, and is much nicer for the client. If we are only working underneath the house, it is highly likely that we can work with you in place. In this case we will seal any gaps in your ground floor to minimize the occurrence of dust, and keep all work areas screened off with our hoardings. Any work noise will be limited to our normal working hours. It would be unrealistic to suggest you wouldn’t notice we were there, but you should be able to continue using your ground floor as usual so disturbance should be minimal.

How effective are modern waterproofing systems. Do they provide a full vapour barrier as well?

Waterproofing technology has come on leaps and bounds since the early cementious tanking systems were used to ‘block out’ the natural passage of water. The Delta Dorken system that OLBC uses is a very robust a plastic cavity membrane system that allows the moisture to run uninterrupted behind the membrane into an engineered sump beneath the floor, which is then pumped away by high powered pumps into the drainage system. This means that no differential pressure is created either in the structure or across the membrane, and also that a vapour barrier as well as a water barrier is created between the earth and the structure.

How do I choose the fit out items for new basement? Do I need to purchase them myself?

Our design team will arrange a series of meetings with you to develop a detailed specification for the entire project. Our role is to introduce you to the design possibilities and to find out what sort of look and budget meets your expectations. We will show you images of various general styles and finishes. We can also provide interior design at all levels of interaction. Use us just to help choose wall colours and tiles, or to design a specific piece of furniture, or for a complete cohesive scheme right down to cushions and artwork.

We will supply all fit-out items, for which we make a small charge to cover the cost of administration, specification, delivery, storage and responsibility for suitability. This means that we become responsible for everything from ensuring everything is correct, suitable, and has appeared at the right time in full working order. It is necessary for us to maintain full control over this aspect of the work in order to provide you with fixed costs and time schedules for completion.