A builder botched an extension at his home, then forged documents enabling its sale to go ahead to two unsuspecting pensioners, who were left thousands of pounds out of pocket.
The couple, in their 80s, discovered the "confidence trick" when they tried to sell the £180,000 house, in Market Harborough, a year later.
Michael Hunter – who works for Leicestershire charity Stride, teaching building skills to people from disadvantaged backgrounds – was jailed for 18 months.
He pleaded guilty at Leicester Crown Court to fraud, by falsely representing that building work at the house was certified as complete by Harborough District Council, between July and September 2010.
After the hearing, the householder said: "We were stuck with the house and unable to sell it. It's been an absolute nightmare and given me sleepless nights.
"It's laughable he's teaching building skills to others, because I wouldn't let him build with Lego.
"My wife and I are both 83 and could have done without all the stress. I'm glad he's gone to prison and only wish it was for longer."
The victim, who did not want to be named, has had to pay £11,365 for remedial work to the botched extension, to obtain a genuine building regulations certificate.
Hunter (51), of Station Road, Kibworth Beauchamp, now faces an investigation into his finances to establish whether any assets can be seized, or compensation paid.
Alan Murphy, prosecuting, said the defendant carried out a loft conversion, extension and installed a conservatory.
He submitted plans to the council.
During a site visit in March 2007, "several areas of concern" were pointed out to Hunter, which he ignored and never contacted them for approval of the completed work.
When the buyers' solicitor requested a buildings certificate, Hunter forged the document on his computer and signed the name of an existing planning officer. The document looked nothing like the real thing.
The pensioners' solicitors accepted it as genuine, no checks were made with the council and the sale went through.
In the summer of last year, the owners discovered the document was bogus.
The fraud wiped up to £50,000 off the house value, until remedial work was done.
The householder said his wife's health had suffered and she had now gone off the idea of moving.
Sentencing, Judge Silvia De Bertodano said: "This was a confidence fraud.
"The complainants ended up trapped in a house you fraudulently sold them.
" I accept you've led an entirely blameless family life apart from this and references speak highly of you as reliable and trustworthy.
"But you forged a document for the purpose of securing a house sale and you derived significant benefit. It's the biggest financial decision most people make in their lives.
"Even if the documents did not look like the real thing, they looked enough like it for a firm of solicitors to accept them as genuine."
Harry Bowyer, mitigating, said: "It was a thoughtless crime, a quick-fix fraud. He never met the victims and did not know they were elderly."
