An incompetent builder whose botched work cost hundreds of thousands of pounds to put right faces a possible jail sentence after being convicted of obtaining money by deception.

Martin Gumbrell, 53, was commissioned by two wealthy clients to build lavish homes, one in the Dorset countryside and the other in Poole Harbour near the exclusive Sandbanks peninsula. But Gumbrell had lied about his qualifications and experience. Neither project was completed.

A court was told that Gumbrell’s lies were exposed when his work crumbled around him. David and Heather Williams paid him £417,000 to build their seaside house, only to be left with a “shell” that cost a further £450,000 to finish.

Gumbrell was a carpenter who had set up his own construction company. Bournemouth Crown Court was told. He told clients that he had a university degree in building, had won awards for his work and was a member of the National House Building Council. His literature boasted that his company specialised in “the art of fine building”.

He told the court: “Gumbrell lied over and over again, treating his victims with callous disregard. He lied about his experience, his qualifications and the quality and quantity of his workforce, leaving a trail of broken promises, shattered dreams and misery behind him.

“He was very much like the O’Reilly character off Fawlty Towers and was a bit of a Walter Mitty because he believed he was a good builder. He is a dreadful man.”

In August 2001 Terry Seaborn hired Gumbrell to build a home on ten acres of land near Sturminster Newton in Dorset. The project was scheduled to last 14 months but four years later it was still not finished. Things went wrong from the start. His first main error was marking out the house too close to a neighbouring bungalow, which meant it had to be replotted.

He then excavated the basement from a clay hillside but failed to put in adequate drainage or shore up the walls, with the result that it flooded and then collapsed. A two-storey workshop built next to the house was then blown down by strong winds because it had been built up too high without sufficient support. Mr Seaborn realised his builder was not the experienced craftsman that he claimed to be when he found himself telling him how to do his job.

Mr Seaborn said: “One of the jokes on the site was, ‘It can’t be Martin’s fault because he has got a degree in building’. He had no idea of supervising a workforce. Things had to be done four times before they were acceptable.”

In February 2004 the Williamses commissioned Gumbrell to build a house in Poole Harbour. The construction should have taken ten months to complete but by October 2005 all that was standing was the shell.

The court was told that he botched the electrics, failed to put a vital support under a balcony and did not secure the roof properly. Rust appeared in an outside wall because he had failed to use a galvanised steel joist there.

In the end the couple sacked Gumbrell and hired professional builders which cost them the extra £450,000.

Gumbrell, from Bournemouth, who has three children, denied that he was unqualified to carry out the projects, saying that he had experience of working for large companies.

He claimed that he had worked on projects including airport buildings and runways in Saudi Arabia and as a site supervisor in Iraq.

He was found guilty of two charges of obtaining a pecuniary advantage by deception between 2001 and 2005. Gumbrell, who was released on unconditional bail, is due to be sentenced next month.