A cellar conversion or basement extension can add valuable extra room to your home without sacrificing valuable garden space. Here’s what you need to know if you’re thinking about adding or enlarging your home at lower ground-floor level

IMAGE: JAMES MERRELL

Basement extensions have become popular with those looking to optimise space in their home, particularly in higher-value areas where the additional complication and cost of building below ground level is justified. It’s also a good option where constraints around the site or planning restrictions limit other opportunities to add more space.

Options include converting or extending an existing cellar or basement, adding new space by excavating beneath the existing property or garden, and even building under the pavement or road, subject to highways consent. Basement extensions can be two or more storeys deep, although the construction cost per square metre increases the deeper you go.

MICHAEL HOLMES

As well as running an architectural and interior design practice, Michael is a TV presenter and author.

The new floorspace – a lower ground floor – is ideally located to provide additional living space, closely linked to the other groundfloor living rooms, but can also be used to provide additional bedroom accommodation subject to compliance with the Building Regulations, especially ensuring that all habitable rooms have windows for ventilation and a safe means of fire escape.

Spaces with lightwells or a sunken terrace are ideal for a family room, entertainment room, or guest bedroom. Basement rooms in the core of the house, with limited or no daylight, lend themselves better for use as a utility room, laundry room, cloakroom, plant room or storage.

Many basement extensions can be constructed under Permitted Development rights —subject to compliance with the rules, which you can find at www. planningportal.co.uk. You could even create a separate basement flat underneath your home, but be aware that an independent self-contained dwelling will always require planning permission.

In this guide, we’ll look at the essentials of basement extensions and how to create a new basement underneath an existing house, but some of the advice will apply to those creating a basement as part of a new build, too.

OVERALL COSTS

Generally, a contractor will price basement work according to three stages or components:

●Shell and core (the structure) including excavation and disposal of spoil

●Waterproofing (determined by the size of the basement space as well as the number of sump chambers and pumps needed)