Basement
conversions
in Barnes, SW13
Architects, Engineers, Surveyors & Builders Under One Roof.
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Richmond allows one basement storey under a Barnes house and keeps at least half of each garden undug. Castelnau's 1842 villas were built with basements; the Edwardian streets were not.
In Barnes the river sets the terms for anything below ground. The neighbourhood keeps its village character within London, wrapped around the pond and the Thames, and its grand Castelnau villas towards the bridge already demand attention to flood resilience. For a basement that means ground and water conditions are read at the very start, and flood-resilience measures are designed into the scheme rather than bolted on — a discipline that shapes the waterproofing, the drainage and the structural strategy alike.
On rates, converting existing space is charged by the square metre of converted area; a new dig with underpinning by the square metre excavated, before fit-out.
One storey, half the garden and the flood zone
Richmond's Local Plan resists any basement of more than one storey below ground under a house, and it defines a single storey as one too low to be split into extra floors later, about 2.7 metres floor to ceiling. Since 1 April 2018 two Article 4 directions have taken away permitted development rights for basements across the whole borough, so a Barnes basement is always a planning application.
The dig may take no more than half of the existing garden land beyond the original footprint, and wherever it runs under the garden the council asks for at least a metre of naturally draining soil above a 200mm drainage layer, so that lawn and planting can return over it.
Flood risk sorts the Barnes streets more than anything else. The plan's basement table refuses new basements, their extensions and self-contained basement flats in the functional floodplain, Zone 3b; in the higher-hazard parts of Zone 3a it allows no bedrooms at basement level; and wherever a basement is acceptable in principle it needs an internal stair to a higher floor and flood resistant construction.
The houses themselves differ. Castelnau's centrally placed 1842 villas are semi-detached pairs, two storeys high with basements, whereas the Village Planning Guidance says of the Edwardian houses that basements were not an original part of their design, and it lists the introduction of basements among the threats to the Castelnau, Mill Hill and Barnes Green conservation areas.
Outside Barnes: Fitzjohns Avenue, NW3, an NW3 house that gained a basement beneath it and a rear extension.
We look up each Barnes address on both flood maps the plan refers to, and find where the roots of street and garden trees run, since the House Extensions guidance says a basement should not harm them.
A pumped device on any waste outlet and a waterproofing design to BS 8102 go into the first drawings, because the plan asks for both.
Sources: Richmond upon Thames Local Plan 2024 to 2039 (adopted October 2025); Richmond: Castelnau conservation area statement (No. 25); Richmond: Barnes Village Planning Guidance SPD (December 2015); Richmond: House Extensions and External Alterations SPD (May 2015).
Basement Conversions in Barnes: Questions
How much of a Barnes garden can a basement take?
No more than half of the garden land outside the original house. The plan also asks for at least a metre of free-draining soil, laid on a 200mm drainage layer, wherever the basement runs under the garden.
Does a basement under a Barnes house need planning permission?
Yes. Richmond's two Article 4 directions, in force since 1 April 2018, removed permitted development rights for basements across the whole borough, so every scheme is a planning application.
Does Barnes's riverside position affect a basement conversion?
It decides whether one is possible. Richmond's plan refuses basements in the functional floodplain and bedrooms at basement level in the higher-hazard parts of Zone 3a, so the flood zone of the exact address is the first thing we look up.
How does Richmond upon Thames control basements in Barnes?
Through Policy 54 of its Local Plan, which limits a house to a single basement storey, keeps the dig within half of the garden land, wants a structural impact assessment next to a listed building and a construction management plan on every scheme.
What structural issues arise digging under a Barnes house?
Water and the neighbours. The plan notes that basements are vulnerable to groundwater through walls, floors and service entries, and it asks applicants to open Party Wall negotiations and talk to neighbours before they apply.
What does a basement conversion cost in Barnes?
The method sets the figure. Converting existing below-ground space goes by the converted area. A new excavation with underpinning is the larger commitment, worked out on the dug area before the fit-out. Where flood resilience and waterproofing near the river are involved, those measures feed into where a scheme lands in its range.
Which Barnes streets and house types suit a basement?
The houses built with a lower floor. Castelnau's 1842 villas are pairs of two storeys with basements, while the Edwardian streets were built without them, and the Village Planning Guidance treats new basements there as a threat to character.
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