Building a garage under the garden

Why the grounds, not the house

A BELOW gallery is a single level, about 4.5m deep, for around 16 cars. It needs a drive-through lift at one end and a drive-out ramp at the other. That takes length, which a large garden or parkland has and a city plot rarely does.

Building under the grounds also leaves the house alone. That matters most for a listed house, where works to the building itself need listed building consent. Our planning permission guide explains the tests.

What shows above ground

• The pavilion. A garden building that houses the car lift. It can read as an orangery, a coach house or a garden room. It is the only structure most visitors will notice.
• The ramp head. The second route out, set into a bank, a walled garden or a change in level, so it reads as part of the landscape.
• Air intake and exhaust. A clean intake in a planted plinth or a pavilion wall, and an exhaust that rises through a pavilion roof or a pergola column.

Approved Document F says exhausts must not discharge into courtyards, enclosures or architectural screens. So vents are designed in from the start, not hidden later. Our air and climate guide explains where they go.

Lawns, parkland and soil on the roof

The roof of the gallery becomes the floor of the garden. It carries a membrane, a protection board and a drainage layer, then subsoil and topsoil. Our waterproofing guide explains how the buried roof is kept dry.

The soil depth follows what will grow on top. A lawn needs less than a hedge or a young tree. We agree the depth with the landscape architect before the structure is designed, because the roof must carry the weight.

Specimen trees

A tree survey comes first, so the gallery can be planned around the trees worth keeping. It follows BS 5837, the British Standard on trees and construction. Each tree gets a root protection area, normally a circle with a radius of 12 times its stem diameter.

The gallery, the working space and the haul roads stay outside those areas. A Tree Preservation Order makes it an offence to cut down, top, lop or wilfully damage a protected tree without consent. Government guidance treats cutting roots as prohibited too. In a conservation area, most tree work needs six weeks' notice to the council.

Ancient and veteran trees are irreplaceable. National policy says their loss should be refused except for wholly exceptional reasons, so the gallery moves, not the tree.

Drainage and groundwater

A buried roof changes how rain moves through a garden. The drainage layer over the roof carries water to the edges, where it is led to soakaways, ponds or the existing land drains. Field drains found during the dig are recorded and reconnected.

Groundwater may need pumping while the dig is open. Small-scale dewatering for building works can be exempt from an abstraction licence if it lasts less than six consecutive months and does not use wells or boreholes. Larger volumes need the Environment Agency's approval.

Building on an estate

An estate usually has room for proper site access, larger plant and a haul road kept away from the house. Topsoil is stripped and stored separately, so it can go back on top.

The care goes into the parkland, the tree roots, any protected species and the lane outside. Bats are fully protected by law, so any old outbuildings or trees in the way are surveyed first. A highway authority can also recover the cost of extra road repairs caused by extraordinary construction traffic, so the lorry route is planned with care.

Restoring the grounds

Once the roof is sealed and protected, the soil goes back in layers, subsoil first, then topsoil. The lawn, beds, paths and any new trees are then laid to the agreed landscape design.

Done well, the only sign of the gallery is the pavilion. Our car gallery security guide explains why that discretion is also the first layer of security.

In London

Some London boroughs set the soil depth by policy. Kensington and Chelsea requires at least one metre of permeable soil above any part of a basement beneath a garden. Westminster asks for one metre of soil plus a 200mm drainage layer.

Both, with Camden, limit a basement to one storey and 50% of the garden. On most central London plots that leaves too little room for a drive-through gallery. Our page on underground garages in London explains where it can work.