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JMCD Developments ultimate guide to basement conversions

JMCD Developments are one of the leading companies in the Halifax and West Yorkshire area for cellar conversions.

We have years of experience working on the most challenging basement and cellar conversions. We carry out cellar conversions for domestic and commercial customers such as home owners and business owners.

A basement conversion might be the best way to add space and value to your home if you don’t have the room to extend above ground. This blog feature tells you all you need to know about how to convert a basement in the Halifax or West Yorkshire area.

Basement conversions are a great way to create extra living space in built-up urban areas where outdoor space for extensions is limited. A basement, naturally closer to the main living areas and garden than a loft extension, is a brilliant way to add more practical living and entertaining space.

There are two main types of basement conversion – the first, and most cost effective – is turning an existing cellar into usable living space. The second is excavating a new basement, which will require more significant structural work and is, therefore, more expensive. Bear in mind that it can be much more cost-effective to convert a basement if you are also adding a new extension above at the same time. However, if you have a small garden and lack living space, both can be worthwhile options.


  • If you need to divert drains beneath your house;

  • If your home has solid concrete rather than timber sub floors

  • If your house is sited on difficult ground conditions (clay, made-up ground, sand or marsh);

  • If the local water table is high, necessitating constant pumping;

  • If access to the site is poor.


Whether it is financially worth converting your basement depends on a number of factors:

  • The cost of the work versus the value added to your home (city-central, high value areas with little available land will give you the best profit potential). Ask a trusted local estate agents for an idea of how much space is worth per square metre in your road. You can compare this to the cost of the basement conversion;

  • If, of course, you are planning an existing cellar conversion rather than a basement dig-out, your spend will be lower, and your profit potential higher;

  • If this is the case and it's a toss up between a planning a loft extension at a similar cost, you'll need to weigh up whether you need more living space or more bedroom space;

  • Check your property's ceiling value with the local estate agent – it's unwise to spend significantly more than you will add to your property's value on any project.

Planning a basement conversion:

Planning a basement conversion is so much more complicated than planning, for example, a single storey extension. However, much of the planning will be done for you by specialists since, unlike simpler extensions, this is not a project that you can easily run yourself. Use these guidelines below to ensure you've got all the planning that you can practically do yourself sorted in advance.


Converting an existing residential cellar or basement into a living space is unlikely to require planning permission. This is provided it’s not going to be used as a separate property, and that the external appearance of the original house is not significantly altered.

Major works to excavate a new basement, adding a separate unit of accommodation, and/or altering the external appearance of your house, are likely to require planning permission, even if you’re converting or expanding on an existing cellar space.

In all circumstances, you should contact your local planning authority for guidance before starting any work. You can apply for planning consent yourself, or employ a professional architect or specialist basement company to apply on your behalf. They will have an insight into local planning regulations and projects in the area that have been successful.


How deep your basement conversion needs to be depends, to a large degree, how you're going to use it. An intimate cinema room or small home gym or utility area will get away with a lower ceiling height than a living space that you want to feel part of the original house.

For the latter use, expect to have to dig deep (around 4m to 4.5m) to get a high ceiling (around 2.7m to 3m). This will also allow you to put in light wells and roof lights so that you get an airy, bright feeling room, despite it being at basement level.


Converting an existing basement from, for example, a storage area into a habitable room can be completed under permitted development (PD) rights, unless you live in a Conservation Area or if your home is listed).

If working under PD, you may want to apply for a certificate of lawful development from your local authority; doing so will give you the paperwork to prove that your scheme met requirements and did not need planning permission.

The general PD criteria is on the government's planning portal, but check with your local council before you proceed, because some areas have more restricted rights.


To ensure your new basement is legally habitable, it will have to meet building regulations by achieving sufficient levels of insulation, creating emergency escape routes and having a minimum head height, which may require structural supports, such as steel beams. Any structural work you carry out will need to be passed by a building inspector, to make sure that it won’t impact on the rest of your house or your neighbours’ properties.


The Party Wall Act provides a legal framework for preventing and resolving disputes in relation to boundary walls, party walls and excavations that are near neighbouring buildings.

Unless you live in a detached house, you will need to reach an agreement with your neighbours over shared walls before converting your basement. The correct notices will need to be issued and consents obtained from all neighbours whose boundaries might be affected at least two months before work begins.

If you are using a contractor to design, manage and complete your basement project, they will usually deal with any party wall agreements that need to be reached, along with planning applications and building regulations. If not, you will need to instruct a surveyor; your neighbour also has the right to appoint their own surveyor, and you will be obliged to pay for both surveyor fees.

If you get your neighbours on board, they may consent to the work and there won’t be any fees involved, but you’ll need to get this in writing on a waiver form.


Tanking is the most common way of waterproofing an underground space. This involves applying a coating to the interior of the porous basement walls to create a waterproof barrier. This can be in the form of a membrane that is fixed to the walls or as some kind of waterproof render or sealant.

The two main methods to choose from are brush-applied tanking or a cavity membrane drainage system. Whichever you choose, products should be British Board of Agreement certified and you should receive an insurance-backed guarantee. Of course, adequate heating and ventilation will also help prevent condensation from forming.


Building regulations require that all floors and walls below ground level are waterproofed, to stop damp entering a building’s structure. Water penetration can be tricky to control; it’s not unusual to find basements where two or three different systems have been applied – bitumen, cellar paint and/or render – but all have failed for one reason or another. Find the right solution for your basement with an assessment by a waterproofing contractor with a proven track record.

For basements where the water table is higher than usual, or in old homes where the cellar walls may be more porous, you will need to have a cavity membrane drainage system. This takes water out of the basement, either naturally or with an electric pump.


Revamping an existing cellar, involving applying a membrane lining system, digging out the sump and fitting a pumping system will take just a few weeks.

A full basement conversion, involving underpinning of the existing house will take several months.


This will depend very much on access: if there is access directly into your garden or the road outside for soil to be removed, and the existing ground floor of your home is constructed from suspended timber, it's likely that you can continue to live there while the basement is converted. 

If, however, your home has a concrete ground floor which has to be removed and rebuilt, you will almost certainly have to/want to move out.

Bear in mind that larger firms will be quicker but possibly more expensive; smaller firms will be less flexible but possibly cheaper in the long run – however the conversion might take longer with them.

Ensure your builder has all the relevant warranties in place, and draw up a watertight contract using a template, such as a JCT homeowner contract. Ensure you are explicit within the contract about when payments will be made: payment upon completion of specific parts of work rather than weekly, for example, is a better route to take. 

If you would like to know more about our cellar and basement conversion services in Halifax and the West Yorkshire area contact JMCD Developments your local experts in cellar conversions.