© ReutersBoris Johnson lays a brick during a visit to a Barratt Homes development in Bedford, last November. The prime minister has backed plans to allow buildings to be repurposed without full planning permission.
Thousands of tiny, substandard "rabbit-hutch" flats could be created in commercial buildings left empty by the coronavirus economic slowdown under planning reforms championed by Boris Johnson.

University College London professor Ben Clifford - who recently completed a government review of housing produced outside the conventional planning system - said allowing developers to turn a wider range of commercial properties into flats without planning checks could lead to a wave of substandard conversions.

"Unless there are proper safeguards, we could see even more poor-quality, tiny flats being crammed into commercial buildings lacking amenities and green space," he said. "These could be what others have rightly called the slums of the future."

Johnson last week pledged to bring forward the most radical reforms to the planning system since the end of the second world war, starting with an expansion of permitted development rights, which allow buildings to be repurposed without full planning permission.

Clifford was last year asked by the government to review the quality of homes delivered through existing permitted development rights, which cover offices, retail and light industrial units. He urged ministers to publish the report, which was submitted in January. "The evidence in the report would help inform a debate that has already started about these important issues, which could lead to huge changes in many towns and cities," he said.

Clifford previously co-authored a report for the Royal Institution of Chartered Surveyors that suggests 70% of flats produced by permitted development are below the government space standards, with some measuring just 15 square metres. Basements with little natural light and blocks on lorry-clogged industrial parks have been turned into flats by developers.

Since 2015 more than 60,000 flats have been created through permitted development in England, with almost 90% coming from office conversions. "It is popular with developers because they do not have to make a contribution to affordable housing and local infrastructure," added Clifford. "It's often far more profitable than going through the normal planning system."

The latest changes will come into force in September. Details are expected later this month but it could cover banks, building societies, clinics, training centres and even gyms. "Under these proposals the government might allow a whole host of other commercial buildings to be converted to residential uses," said Clifford. "It could lead to thousands of new conversions."

Some areas have already seen a flurry of controversial office to residential conversions. In Harlow, Essex, half the new homes created in 2018/19 came from office conversions. The town's Conservative MP Robert Halfon said he would be writing to the prime minister and the housing minister, Robert Jenrick, about the latest proposals. "I think it is a disaster in the offing," he said. "If there are not proper controls on quality and developers are allowed to build ghettos for people on the lowest incomes then we will have a repeat of what we've had in the first wave of permitted development."

Halfon called for Clifford's report to be published. "I'm in favour of liberalising the planning system" he said. "But permitted development is about quantity rather than quality."

Hugh Ellis, policy director for the Town and Country Planning Association, accused ministers of sitting on Clifford's report because it did not align with their deregulatory drive. "The report will - I'm sure - show that permitted development has produced some dreadful outcomes," he said. "Could it be that the government received a report raising very significant challenges about what they have been doing and they choose to ignore it?"

According to the Local Government Association, more than one million homes granted planning permission have not been built by developers in the past decade. "Planning is not a barrier to building new homes," said David Renard, the LGA's housing and planning spokesperson.

A housing ministry spokesperson said: "Like any other project, homes built under permitted development rights must meet rigorous building regulations. These new regulations will cut red tape and unnecessary bureaucracy while maintaining high standards, and make effective use of existing buildings in keeping with the character of their local area."