New planning rules which make it harder for developers to backtrack on promises over affordable housing have been hailed as a chance to “ease the roadblock” holding back new homes in the city.

In a speech on Monday, Prime Minister Theresa May unveiled proposed changes to planning policy aimed at ensuring more houses will be built in coming years.

The plans would force developers to provide a set minimum number of affordable homes, and force councils to sign up to a nationwide standard showing housing need.

Crucially for Brighton and Hove, the proposals would stop developers using “viability assessments” to backtrack on commitments on affordable housing.

Buried in 150 pages of plans from the Ministry of Housing and Local Government the proposals state: “Where a viability assessment is submitted to accompany a planning application this should be based upon, and refer back to, the viability assessment that informed the plan; and the applicant should provide evidence of what has changed since then.

“Potential risk is accounted for in the assumed return for developers at the plan making stage. It is the role of developers, not plan makers or decision makers, to mitigate these risks. The cost of complying with policy requirements should be accounted for in benchmark land value.”

The proposals would also reduce the amount of profit developers can expect to make on a scheme from 20 per cent to six per cent, in certain cases where affordable housing is particularly needed.

The changes would make it harder for a developer to come to the council – as many have in recent years – and claim it cannot afford to provide the number of affordable units the council needs from a scheme.

The plans received broad support. Councillor Julie Cattell, chairwoman of Brighton and Hove City Council planning committee, said: “If she can bring that law in and get developers to drop their expectations that’s great.”

She added that a city rule change which now forces developers to provide their projected profits under “open book” planning rules might “shame” some into cutting into their own bottom line to build more affordable units.

Her Conservative opposite number councillor Carol Theobald said: “I think it will really help. This has a chance of easing the roadblock to affordable housing in Brighton and Hove.

“We’ve had so many applications where they’ve been passed and not built and there’s so many houses needed.

“I think it’s quite right to ask developers to make less profit.”

Green councillor Tom Druitt said: “With this news now from the Government, the door is open for the city council to insist on our policy of a minimum 40 per cent affordable housing in all new developments and finally address the housing crisis in the city.”

Liam Broadfoot, who has been looking for somewhere in Brighton to buy with his partner Becky for months, said: “Forcing developers to make some of these new houses affordable will really give more young people a chance to get on the ladder.”