Britain is seeing a boom in the building of micro-homes with some little bigger than hotel rooms or prison cells.
About 7,800 were built last year with many converted from city centre office blocks.
However, the consumer group Which? is warning that some banks will refuse to lend on the tiny properties amid fears they could be difficult to sell on.
Research by the group also found that micro-homes do not increase in value at the same rate as others.
Britain is seeing a boom in the building of micro-homes with some little bigger than prison cells. Micro-property typically has a floor area of less than 37 sq m
A micro-property has no strict definition, but typically has a floor area of less than 37 sq m – the minimum size for a studio under Government standards.
In comparison the average UK car parking space is 11.5 sq m.
These standards are not a legal minimum and the Government relaxed planning rules in 2013, making it easier for developers to convert offices into homes under permitted development rights.
A total of 7,809 micro-homes were sold in 2016, according to the Land Registry. That represents an increase of 40 per cent on 2015 and 172 per cent on the 2,871 of 2014.
The booming housing market has made it impossible for millions of young people to buy a home. Squeezing more homes into a block does bring the price down, albeit at the expense of living space.
However, these micro-homes are not necessarily cheap. In 2016, properties in London smaller than 37 sq m sold for an average of £279,000.
Which? said it has found one in Brent, north London, that was just 8 sq m.
A home of that size would be only 1 sq m larger than a prison cell. The average hotel room in a Travelodge is 28 sq m.
However, Which? said many newly converted office developments offer apartments with floor areas of 16 sq m, smaller than a sumo wrestling ring.
For example, there are plans to turn Barnet House, an 11-storey office block in north London into hundreds of tiny studio flats, some at 16 sq m.
In Central Cross, Croydon, south London, a company is selling 31.5 sq m, one-bed apartments with high-end fixtures, for just under £283,000.
Which? said high numbers of micro-properties are also being built in Leicester, Liverpool, Cambridge and Bristol.
Land Registry figures show price growth on micro-homes from 2013 to 2015 was 6.9 per cent, versus 8.7 per cent for larger properties. The increase for homes with less than 30 sq m was lower still at 5.4 per cent. It may be because some lenders will not offer a mortgage on very small homes.
The average hotel room in a Travelodge is 28 sq m. One property in Barnet was found to be just 8 sq m
Nationwide and RBS won’t lend on properties with floor areas smaller than 30 sq m.
The charity Shelter, which campaigns for more homes, said: ‘In theory, converting offices sounds like a good way of creating some of the homes we need in a crisis.
‘But in reality, it’s often used by developers as a way of cashing in on people’s desperation by building unaffordable, rabbit-hutch homes.
‘Our homes are already among the smallest in Europe, and if the Government allows developers to cut corners by slashing space standards in homes even more, they will be punishing ordinary families in the process.’