Twenty-six studio flats measuring as little as 18 sq metres – that’s just under 14ft by 14ft for residents’ entire living, washing and eating space – are set to be crammed into a scruffy two-storey commercial building on an industrial estate. A typical Premier Inn hotel room, by comparison, is 21.3 sq metres.

Some of the flats would apparently be windowless, with the only natural light seemingly coming from a roof light or skylight – so they would offer no view out.

A leading architect says the plan to convert an office in affluent Balham, south London, into tiny flats is the latest “shocking” example of the controversial phenomenon known as office-to-residential conversion. Across the country, commercial buildings are being converted into often minuscule flats – using new laws that enable developers to bypass the traditional planning permission system and national space standards.

There would be no private or communal garden space at the Balham development. And because you would be living on a busy industrial estate, it’s claimed that just a short distance from your front door you will have heavy lorries coming and going, cutting equipment being used, forklift trucks moving items around, and businesses that start work as early as 4am and don’t shut up shop until 11pm. So you might want to think twice about letting the children play outside.

Julia Park, who is head of housing research at architects Levitt Bernstein and one of the London mayor’s “design advocates”, claims that if the conversion is allowed to go ahead, “the risk to the wellbeing – mental health in particular – of any occupants would be considerable”.

And, with the plans indicating that four of the proposed flats have no vertical windows at all, she adds: “Our prisons do better than that.”

During the past few years there has been a frenzy of office-to-residential conversions in some areas – a boom that is largely down to the fact that in 2013 the government relaxed the rules relating to office buildings being turned into housing.

Many of these new flats fall far short of national space standards, which say the minimum floor area for a new one-bedroom one-person home (including conversions) is 37 sq metres, and for a one-bed two-person home is 50 sq metres. While these minimum sizes are not compulsory, they do apply in London – but only to schemes that go through the planning system.

That’s the other thing that is so controversial: under so-called “permitted development rights” (PDR), office-to-residential conversions don’t require planning permission – the developer simply has to notify the local authority of its intentions and gain what is called “prior approval”. This effectively means planning approval has already been granted by the secretary of state – but without seeing any of the actual proposals.

So what’s the deal with the Balham scheme? Plans were originally put forward for three office units at Grange Mills industrial estate to be converted into 13 flats, and these were accepted by Lambeth council last November. But in December, Caridon Developments, based in Croydon, south London, applied for permission to squeeze 26 self-contained studio flats into the same building.

The office units are currently occupied by a holiday company. They were built before 1945, and have a corrugated steel sheet roof.

Lambeth council has received several objections to the plans from local people and businesses, one of whom said that in traffic terms, “this area is an accident waiting to happen”. Another pointed out that some of the firms on the estate operate long or antisocial hours.

Two of the planned flats are 45 sq metres, but more than half are between 18 and 20 sq metres. The floor plans arguably leave a lot to be desired in terms of detail, but Park says they seem to indicate that with four of the ground floor flats, the only windows look into shared lightwells, while on the first floor, four studios “appear to have no vertical window at all – three rely on a long skylight which stretches across five dwellings, [while] the fourth has a single roof light”.

She says: “Building regulations have never required a living space to have a window, not because it doesn’t matter, but because no one imagined that anyone would offer a home without one. Now we know that has become a reality, we need to legislate to prevent it.”

Lambeth council told Guardian Money the application would be decided by council officers under delegated powers, and that approval is subject to a “section 106” legal agreement. However, the council clearly isn’t a fan of the PDR policy, which it says results in developments “which do not offer any affordable housing, do not provide contributions to fund infrastructure, and often result in poor-quality housing”.

Park goes further, saying PDR for office-to-residential conversions “has created some of the worst new housing seen in Britain for decades” and needs to be ditched by the government.

In an article last August, Money highlighted the example of a former office block next to the A12 in Ilford, east London, which has been converted into 60 rental flats that were being used by Redbridge council for temporary accommodation.

In a report published in May 2018, the Royal Institution of Chartered Surveyors said its research into office-to-residential conversions had uncovered some examples of “very poor housing”.

Caridon Developments was contacted by Money but declined to comment. On its website, Caridon says of its developments arm: “We identify projects, take them through the planning process and build quality accommodation which delivers impressive return on investment.”