Alex and Maxine Mamas, the proprietors of the Clerkenwell-based ironmongery firm, have issued a High Court writ against the Sevenoaks practice.
The Mamas are demanding damages of nearly £400,000 and catalogue a series of problems with their new home.
The High Court writ says they engaged Barrett Haskins Design to design a new six-bedroom home with an indoor swimming pool at 18 The Meadows, Chelsfield Park, Orpington, Kent, and paid the company £41,283.
But they now claim that the designer failed to tell them to take out site insurance; certified money as being due to contractor GJ Cole Partnership without properly checking the value of the work carried out and materials supplied; certified work which was defective; and failed to administer the building contract to achieve speedy and economical completion of the project.
The Mamas say that the practice certified that the work was worth £692,374 when it was instead worth only £622,746 - a difference of £69,627 which they paid to the building firm, which then went into insolvent liquidation.
They were forced to call in another firm to finish the work, and they claim this cost another £514,614 instead of £202,158 allowed for in the original contract, a difference of £312,456.
The Mamas also complain that drains, plumbing, electrics, and heating in the new home weredefective, and had to be redone, that pipes had to chopped out, other pipes leaked, and that three first-floor rooms had no earth connections.
They lost more items worth more than £6,000 in a burglary, and claim they suffered inconvenience, distress and loss of enjoyment by having to live in their old, three-bedroomed house which they had planned to demolish.
The Mamas also say they had the inconvenience of having two or three staff present in their small house, and had to share their bedroom with their two younger children.
The project should have been finished by November 2001, but by the time the original builders went into liquidation in August 2002, the scheme was not finished, the writ says.
The house is still not finished, but the Mamas say they accept that Barrett Haskins Design is not responsible for the whole length of time the project has taken. Instead, they say the work should have been finished in a year.
They are suing the architect for £6,596 for the burglary, £69,627.26 for the difference in value of the certified works, £16,000 for the cost of rectification work, the increased cost of completion of £312,456, return of the fees paid to the company, and damages for inconvenience and loss of amenity.