Extending Below Ground – Adding a Basement

For many the thought of moving house in this economic climate is simply out of the question. But if you need more space and your garden isn't exactly going to accommodate the size extension you require then you might just have to think laterally – or is it vertically?

Planning issues often make adding an extra floor to your home impossible. Homes which are listed or in conservation areas will not usually receive planning permission for any obvious extensions. So the only option left is to consider an underground extension. Out of sight and out of mind of the neighbours, this type of extension is finding favour with those who want something which will not affect the look of their home. Of course the planning department will still need to be informed and if there is any alteration to the outside of your home planning will be needed.

It is important to check with your lawyer that you own the property under your home. Sometime the land can be on a long lease. A surveyor will also need to be instructed to check the sewers, phone and electrical lines. A damp proofing expert will be required and and underpinning specialist. All these people will need to be in place before you start.

The usual party wall permissions must also be obtained unless you live in a detached home. In addition to all this you will need the advice of a structural engineer to ensure that your foundations can support all this extra work and stress.

Unfortunately digging a basement extension is going to be more expensive than the conventional type and can cost upwards of £300 per sq ft compared to around £100 for a loft conversion. It may be worth considering the fact that this type of extension may not make you much return on your investment, however if you plan on staying in your home then this won't really matter.

The older your home the easier it is to dig out the foundations due to the way they are constructed. They tend to have deeper foundations rather than pilings. It is also easier if the ground floor of your home is suspended timber rather than concrete. A concrete floor will have to be dug into, while those with a timber floor can have the basement dug from the outside.

You can expect the digging of a basement to take 8 to 12 weeks, once planning permission has been obtained. After this the decoration will need to be done. So in all you could be looking at anything up to 6 months. If you need to move out of the home while the work is done then this needs to be added to your costs.

Finally you need to consider where the light for your basement will come from. In some cases one end can open to the garden depending on how much you are willing to dig out. In other cases a light well or even a glass floor to the upper level can be installed. In some cases the basement may be used as a gym or swimming pool area and the need for natural light may not be as great.

Here's the scary bit – the cost. Architect, surveyor and engineer fees can be as much as £20,000. Planning permissions can cost £4000 and the build cost could be as much as £16,000 for underpinning and up to £23,000 for the digging of the basement. Finally the costs of tanking the walls, adding a staircase, plastering and decorating can cost around £24,000. So put aside at least £75,000 but expect it to cost more. Of course a good DIYer can do some of this, but experts will be required for the specialist jobs.

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