The media has recently been awash with stories of contravertial super-basement conversions - we either love them (if we're the home-owner) or we hate them (if we're the neighbours) and there is little room for commentary in the middle. As they continue to make the headlines, we republish one of our most well-read blogs from last year, which offers an unbiased view on the pros and cons of extending property square footage by digging down.

London is awash with basement conversions and has been for the last few years. Now it seems the trend for building downwards has spread beyond the M25.  Perhaps initially used as a recession defying way to create more space as an alternative to up-sizing, basement extensions seem to be the go-to option for creating more square footage and adding value to your property.

Why build up when you can build down?

Experts have been quoted as estimating that by spending £300 per square foot on building a basement, you can expect to yield as much as £2,000 per square foot at the point of sale. With that rate of yield, and space at a premium, it’s easy to see why basement excavations are becoming so popular. However, basement extensions have not been met with universal support, with many high profile local residents protesting against the rise of the ‘iceberg homes’ and the constant disruption they cause (I’m not even going to get started on the various horror stories of collapsed basements). It is not only the disruption to your neighbours that you must consider when you decide to expand downwards: unfortunately it’s not as simple as digging a really big hole in the garden. Below I have set out some of the things you should consider when doing any substantial work to your house, but with a particular focus on basement works.

Planning permission

If you are converting an existing cellar into a habitable basement space, you may not require planning permission, as your conversion will simply involve a change of use. Similarly, some basement works are covered by permitted development rights. However, if you are creating a new basement which involves major works, a separate unit of accommodation and/or alters the external appearance of the property, e.g. adding a light well, you are likely to require planning permission.

Kingsley Napley would recommend that you or your architect contacts the local authority before you start work for guidance on whether you will require permission. If you do not require permission, it may be worth requesting written confirmation of this, as this will be useful when you come to sell the property on. You should remember that you may also require other permissions, such as from the freeholder, or estate holder. If you live in a listed building you are likely to need consent for any works done.

Those that live in listed buildings or within conservation areas will have even more hurdles to climb.  Listing buildings and conservation area are buildings or areas of special architectural or historic interest, the character or appearance of which it is desirable to preserve or enhance. As such, these building and areas have extra planning controls and there is less permitted development as a result.

Building control

Even if planning permission is not required, if you are creating a new habitable space at your property, Building Regulations approval will be required.  Building Regulations are separate from planning permission, and provide statutory minimum construction standards to ensure that buildings are safe, hygienic and energy efficient. If you are looking to sell the property on once the works are finished any perspective purchasers will want to see a copy of the Building Regulation Completion Certificate. If you do not obtain one, you may be asked to cover the cost of an indemnity policy for the purchaser, or in worst case scenarios, a sale may fall through due to the lack of building regulations approval, with a lender refusing to advance monies on a basement which has not been signed off as structurally safe.

Party walls

If the property is a terraced or semi-detached house, you may well be excavating or building near your neighbours’ walls, in which case you must be sure to comply with the Party Wall Act 1996. Party Wall agreements are required in three circumstances:

  • when you build a wall on a land boundary;
  • when you do work on an existing boundary wall; and
  • most relevant for our purpose, when you are excavating near a neighbour’s building.

Notice must be given in the following circumstances:

  • when you’re excavating within 3 metres of the adjoining owner’s buildings
  • if the proposed foundations are going to be deeper than the neighbour’s existing foundations, or
  • when excavating within 6 metres if the proposed structures would be dissected by a line going 45 degrees from the base of the neighbour’s existing foundations.

If any of these circumstances apply, then a month’s notice must be served on the owner of the adjoining land. This notice remains valid for one year only and you must set out your proposals for your excavation work and details on whether you are planning on reinforcing your neighbour’s existing foundations.

The requirement to notify the adjoining landowner is also triggered if you are going to be excavating near ‘any part of a building or structure’. While the term ‘building’ is fairly easy to identify, the term ‘structure’ has been widely interpreted by case law to include a wall, and therefore our advice would be to seek professional advice prior to commencing any excavation works. 

The future for basement conversions

If, like me, you are an avid watcher of Grand Designs, you will know this is only a very small snapshot of the issues you should consider before you start digging. A host of practical, design and logistical considerations will need to be balanced with the legal processes I’ve outlined above. These processes could become even more stringent in future; a bill called the ‘Subterranean Development Bill’ was introduced to Parliament in 2012, although it never got further than a second reading at the House of Lords. Given the repeated calls by local residents and politicians for legislation to be put in place to govern more closely what can and cannot be done, it seems that it is only a matter of time before we see further legal developments in this area.