Customers allege a building firm, which abandoned a project in Costessey after being paid £43,000, has also left them thousands of pounds out of pocket.
Sam Elmhirst paid an Essex-based company called Hudson Garden Rooms £43,000 to build the granny annexe in her garden in Myrtle Avenue.
But she was left with an unhabitable lodge last year which one surveyor estimated needed £37,500 of work to fix.
When she tried to take the firm, Hudson Garden Rooms Limited, to court last year, it folded with almost £400,000 of debts.
But the owners of the firm, couple Chris and Sophie Escrader continued to trade under a new firm called Hudson Living Ltd.
An investigation by the Romford Recorder has now found customers handed over tens of thousands of pounds to Hudson Living and Hudson Garden Rooms for incomplete work, but are unable to contact the Escraders.
Alan Street, 36, from Hornchurch, paid £44,000 for a botched extension job,
He now has an empty timber shell in his back garden and faces the prospect of spending even more to fix the mess.
Unknown to him, the company had gone into liquidation with debts of almost £400,000 by the time he paid a deposit in December 2018.
"They didn't even bother to get building regulations," they said. "To all intents and purposes it looks to me as if they have disappeared."
Iqbal Channa, from Lewisham paid a £5,000 deposit in July 2018, followed by another £40,000 in instalments, for eight days' work that did not get under way until April 2019.
Mr Channa's build was also abandoned, with tiles piled up against the wall and the family no longer able to access their back garden except through a side door.
He later discovered that the company had never obtained planning permission for it in the first place.
He said: "We can't actually access our back garden now. It's just depressing. Even more depressingly, there's no protection [for victims]."
A couple on the Essex coast, meanwhile, said they paid £49,000 for a granny annexe.
"The whole thing looks like an amateur build," they said.
The official website for Hudson Living is still live, but calls to its number are forwarded.
The Escraders did not respond to requests for comment.
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