Property Blog and News / Basement conversions: 11 questions you need to ask if you’re planning on digging down

Basement conversions: 11 questions you need to ask if you’re planning on digging down

If you have extended up and out, now’s the time to turn your attention downwards.

The good news is basement conversions can add between 10-15 per cent to your property’s value, according to Savills.

They can add much needed living space, or even be rented out as a separate flat, making them a nice little earner on the home front. More visionary homeowners (with deeper pockets) might opt for underground parking, cinema rooms or gyms, making their properties iceberg-like, with more space below ground level than above.

“Basements, when done correctly, can add much needed space to a family house which has been extended to the limits above ground, not to mention some serious value,” says James Davies, Head of Knight Frank’s Fulham Office.

“Over the years, I have seen some superb – and some dreadful – basement arrangements. From incredible wine cellars to a climbing wall for the children, all you need is a bit of imagination to create something amazing.

“It is easy to fall into the trap of cutting corners as costs mount, but – ‘digger beware’ – that slightly lower ceiling height or the quick and easy drainage solution can come back to bite you.”

Here is our round-up of everything you need to know when considering a basement conversion.

1. Is it worth it?

It might be in London where the cost of moving to a bigger house outweighs the cost of converting a cellar or digging a basement level. An existing cellar could be converted for less than £50,000, but more extensive designs can cost £500,000 and more.

A loft conversion would probably cost half as much but with a basement you are creating living space rather than bedrooms. Or you could create a separate entrance from the street and let the basement out for extra income.

“Basement extensions have become the norm in London as an answer to the capital’s building space shortage,” says Ewan Stoddart, owner of EDWS Architects.

2. Do I need planning permission?

Not if you are simply converting a cellar into living space. If you need to excavate the floor to provide more headroom, you may need planning permission. James Davies adds: “Ceiling height is crucial to optimise subterranean space, ensuring the proportions of the room feel normal.”

If your house is listed, or is in a conservation area, you will need permission and in every case you need to comply with building regulations. James Davies explains: “Planning here in Fulham, given the number of basement digs being done at any one time, doesn’t appear to be too taxing as long as you aren’t pushing the envelope too far from what ‘the norm’ is for the area.”

If the work will affect a party wall in any way, you must inform your neighbours and any leaseholders of adjoining properties. Seek advice if you are unsure, especially in London, where some neighbours have taken legal action to prevent further excavation of their streets.

3. Will it be damp?

Some cellars are damp because of a lack of ventilation and some are damp because water courses run below them. Various measures can be used to prevent moisture but it is invariably better to consult an expert.

Get this right, because nobody wants to spend time in a damp basement. Check your flood risk too. If it’s high, it might be wise to take extra steps to avoid any potential for flooding.

4. What about ventilation?

Many cellars are poorly ventilated and that is why they smell dank and musty. If you don’t tackle this problem, carpets and soft furnishings may go mouldy. Windows should be opened wherever possible, but if the problem persists, you may need to use a dehumidifier or a built-in mechanical ventilation system.

You will want to extend your heating system to include the new basement rooms, so you will need to check that your boiler is able to cope with extra radiators.

5. Will we have to move out?

It depends on how much work needs to be done. If you have solid floors and no existing basement, the work will be substantial and you will almost certainly have to move out.

If there is external access to an existing cellar, you can probably stay, if you don’t mind the dust and noise. Remember, that your neighbours will not have the choice, so be considerate to them.

6. What hidden costs might there be?

If the walls need underpinning, services need moving, or access is poor, the costs will increase. Interior walls may be load bearing, so do not remove any without expert advice.

You need a minimum height of 2.4 metres, so if the space will be less than this after you have installed damp proofing and flooring, you will need to dig down.

7. Do I need to ask the neighbours?

If the work will affect a party wall in any way, you must inform your neighbours and any leaseholders of adjoining properties. It’s best to approach your neighbours informally first and get them onside.

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They might be annoyed if the first they hear of it is when they receive an official notice. Drop round and explain your plans, then engage a qualified party wall consultant to draw up the paperwork.

James Davies, Head of Knight Frank’s Fulham Office, adds: “I recently visited a house where both properties either side had done basements within the last few years, meaning that the foundations were 90% prepared for the middle property.”

8. Will it be dark?

Good design will be needed to avoid a gloomy space. If you are only going to use the area as a utility room or storage area, light is less important, but if you are planning a living room, kitchen or bedroom, you will need to think carefully about ways to bring in natural light.

Light wells skylights and glazed ceiling panels can increase levels to the extent that you will never know you are below ground. If light is an issue, consider using the space as a cinema room or even indoor swimming pool if the budget allows.

9. What about drainage?

Installing a shower or bathroom and loo will involve some interaction with the plumbing and sewage systems. The drains may have to be diverted and in some cases pumps installed.

You might want to design basement rooms with an eye to where similar services are located on the floors above. The fewer bends needed in the pipework, the easier and cheaper the work will be, with less chance of leaks and blockages.

10. What about access?

If it’s a space being excavated as opposed to a cellar being redeveloped, you will need to consider access for machinery such as a mechanical digger. You also need to plan where the stairs will go to fit in with your ground-floor layout.

Usually, access to the garden is fairly easy to plan, with steps leading up or even a courtyard level with the basement rooms. Glass bridges or platforms can help allow light into the new rooms without losing any outside space.

11. Who can I speak to for advice?

If you are thinking seriously about a basement conversion, take advice from your local estate agent. James Davies, Head of Knight Frank’s Fulham Office, says: “In terms of what to add, clearly this depends on your needs, but try to safeguard your investment.

I would always suggest sticking to the fairly well-trodden path of: additional bedroom with en suite, or a laundry room and a versatile family/media room. If you have additional space, then one wall as a wine cellar (or even an entire separate room) is a show piece, not to mention useful.”