The wet room-style bathroom, with toilet, sink and shower, is separated from the kitchen by a sliding glass door.
But there is no privacy for its occupant if someone is using the kitchen at the same time.
There are lots of other space-saving tricks that aim to make the small space a liveable home.
The front door opens into the kitchenette which has built-in units, a fridge, a sink, a hotplate and a microwave combination oven.
The rest of the room is a 9ft 2in by 7ft 3in living space which acts as a living room, dining room, bedroom and study.
There is a sofa along one side with a double bed that pulls down from the wall above, and a shelf which can act as a TV stand, dining table and work space.
Above that is storage and to the side is a mirrored cabinet big enough to hang clothes in.
Architects Ian Hogarth and Claire Farrow, who are husband and wife, bought the cupboard for £120,000 in 2007 before converting it and renting it out for £875 a month.
It now up for sale for £225,000, equivalent to £1,991 per square foot.
The property is less than a third of the size of the national minimum new-build space standards but because someone had started living in the flat before the couple bought it, its right to be used as a residence had already been established.
Farrow, 49, said when they bought the property its walls were sticky and covered in grease.
“But these tiny, tight spaces really excite me because I know Ian can work wonders with them. Housing is desperately needed in London, but just because it is small doesn’t mean it can’t be luxurious,” she added.
Hogarth, 62, said the duo had to make the property user-friendly and employ ways of making it seem far more spacious than it really is.
“Whatever we are designing, especially in London, we are always up against the problem of space,” he said.
“This is the smallest space I have ever had to work in but it’s the same principle I apply to larger projects – maximise every last inch.”
Hogarth’s number one rule is to “make everything work at least twice”.
“It’s a waste to give a space just one function no matter how big your house is,” he added.
The couple have put the studio on the market with Haus Properties and are now turning their focus to other projects.