How to plan a basement extension

Planning permission, flood risks, costs and timelines - expert advice on planning a basement extension

By Rebecca Foster | 1 July 2020

Considering a spectacular subterranean space to maximise your home’s potential? Dig deep into this guide to planning a basement extension and find out how much one costs.

Adding a basement level can be a useful route to gaining extra rooms, particularly if you’ve run out of possibilities to extend above ground or want to avoid encroaching on precious outdoor areas.

‘The best place to start is with the possibility of gaining planning permission, rather than anything technical,’ says Sophie Determann, senior architect at Hut Architecture. ‘Check whether there are any constraints on your home – is it listed or in a conservation area? These sorts of things may affect what you can do, and the permissions you need.’ It is also worth checking whether any of your neighbours have carried out a project that can be used as a planning precedent.

Assessing the site

In terms of construction, one of the limiting factors that can affect your plans is water. The presence of a high water table can bump up costs, so check the Environment Agency’s flood map to find out if your home is in a flood zone.

Having good access to the construction site is a common issue that crops up. If you live in the middle of a terrace, excavating beneath your home and transporting the soil and rock away from the site will present more of a logistical challenge than a detached house.

As part of the design process, your architect is likely to appoint a structural engineer. ‘There will be numerous studies to establish important features that may affect the build such as the underlying rock, the ground conditions and the flood risk,’ continues Sophie. ‘There will be some investigation on site, including digging trial pits, to establish what the foundations are and where the water table is.’

If your house already has a cellar, it may be possible to convert this into living space. ‘A cellar will either be tanked [waterproof render applied to the walls internally] or lined with a water management system,’ says Sean Ronnie Hill, director at RISE Design Studio. ‘The latter is generally the preferred solution. Water ingress is allowed to enter the property and is then channelled back out using a specialist drainage system and internal membranes.’

Basement extension view out to garden access - extensions - grand designs

A stepped planting scheme creates a view from the basement. Whitewashed exterior walls bounce light into the room. Photo: Matt Clayton

Planning permission

While it is possible to carry out projects under permitted development rights, instances of such schemes are becoming increasingly rare as authorities clamp down on subterranean builds. In some London boroughs, this type of project automatically requires planning consent.

‘Councils have specific basement policies, so it’s important to make sure these are understood from the start,’ says Ian Hogarth, director at Hogarth Architects.

Obtaining permission will depend largely on the depth of the basement, the layout and whether any changes are being made to the exterior of the property. Building a light well to channel light down to a lower level automatically requires formal consent.

‘It’s advisable to appoint an experienced architect to navigate all that is required to gain permission, as there are several factors that need to be addressed,’ says Sean.

Allocate enough time in your schedule to allow for any issues or delays during the planning phase. Basements are more complex projects than straightforward extensions or loft conversions, so it’s vital to engage the right team of experts from the outset