“It’s going to have a major cumulative effect on the way water drains to the
Heath. It may also affect neighbours’ gardens and even their foundations.
There’s also the general noise and disruption of that kind of building work.”

He added: “Why build a gym when you can just go and run around Hampstead
Heath?”

There is already a basement at Mr Law’s property but this will be extended by
excavating under the back garden terrace for the entire width of the house
to create a ‘lightwell’ and gym area.

The £8m house is part of a terrace so the basement extension will reach the
property lines of the plot of land. The Sherlock Holmes star also wants to
create a sunken garden next to the new gym and to add solar panels to the
roof.

The 1832 property will also boast a wine cellar.

The plans also aim to remove certain modern features added in recent years to
“return the house to its original splendour” including a staircase, ‘modern’
doors and renovating the fireplaces.

Mr Law’s architect, Richard Keep, said the changes are justified as the
current design ‘does not reflect the original layout’ of the building.

A source close to Mr Law said: “If permission is granted this involves a
minimal amount of digging and nothing will happen that will adversely affect
the environment and the surrounding area.”

Mr Hammerson said the society was also encouraging anyone building in the area
to undertake historical excavation as part of any works.

He said: “The area Mr Law will be digging will have fairly significant
archaeological implications for the local area, some of those houses date
from the early 18th century and there may well be valuable archaeological
information from that period or even earlier dates.

“It is important that we preserve that history,” he said.

“There is, of course, an open invitation to Mr Law to come and talk to the
society about our concerns.”

Mr Law, who has three children from his marriage to fashion designer Sadie
Frost and a daughter from a short relationship with American model Samantha
Burke, bought the house in November while he was still dating Miss Miller.