Underground dam fears spark calls for Hampstead basement map

The threat of millions of pounds worth of damage to Hampstead properties has led to calls for a basement map to be drawn up.

Uncontrolled basement extensions could see underground dams form under the conservation area, altering subterranean water courses and washing away fragile surface soils affecting foundations, the Heath and Hampstead Society claims.

The society and Camden Council are in talks to draw up a map of the underground developments and soil conditions to highlight the risks of unchecked basement developments.

The society’s planning sub-committee chairman Gordon Maclean said: “Dams could indeed build up in some circumstances and certainly the deeper you go the worse it gets.

“It (the basement map) is part of our ongoing campaign involving basements, but we have not reached a stage where we have decided where the map’s boundaries should go.

“It would be easy to apply it to the whole of the borough, but we’re trying to narrow it down where we know homes are at risk.”

The emergence of an unexplained underground cavern in Redington Road has renewed fears of the effects of unregulated basement developments in Hampstead.

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Two neighbouring properties have already had basement extensions built along the road and proposals for a nearby third are set to be considered by planning officials.

A council spokeswoman said there was no clear evidence about what had caused the subsidence, but said the most common cause was water washing away the top level of soil.

She said: “As the void was dry it meant it could have lain dormant for a long period of time before the top surface fell in. Fortunately incidents of this nature are isolated and rare.”

Hampstead Town Cllr Chris Knight said: “Once you have put something in the path of a water course it has got to go somewhere and if you put things in a line to block it, a dam is going to form. It will cause problems upstream and downstream.

“The map would be an immediate indicator for residents and planning officers where basement works have previously taken place and hopefully highlight where adjoining developments could risk turning into a dam.”

Currently planning officials only consider previous developments when a planning application is lodged with the council.