The U.K.'s development planning process is notoriously labyrinthine, but the new rules should remove around six months to one year from the timescale, global law firm Eversheds estimated.
"This eliminates one stage of argument and focuses on technicalities needed to deliver a scheme,"Stuart Andrews, partner and head of planning at Eversheds, explained.
"The ability to change just in principle has freed up a lot of buildings, even in locations where there's restraint."
The focus of much development in London has been at the super-high end of the market, rather than at the quantity-rather-than-quality, middle-income bracket.
(Read more: Singaporeans continue their love affair with London property)
"This could result in some substantial changes of use in certain locations, where there are old Victorian warehouse buildings still in warehouse use," Roger Hepher, Savills' head of planning and regeneration, said.
The trend could also catch on in other major U.K. cities, like Manchester and Birmingham, once their economies start to accelerate further.
The Conservative Party, of which both Johnson and U.K. Chancellor of the Exchequer George Osborne are leading lights, is usually supportive of the property industry. However a number of Conservative members of parliament (MPs) are strongly against building on the so-called Green Belt – countryside land that is protected against development – and this latest measure could be one way of the party allowing more housing while keeping its backbench MPs on side.
- By CNBC's Catherine Boyle. Twitter: @cboylecnbc.