John had agreed to build four detached houses for kevin, a developer, at a cost of £500,000 each. it was agreed that each house would be paid for when it was completed and that the completion had to be certified by lara, an architect. john completed two houses but kevin refused to pay because, he claimed, the windows were not up to specification. lara died just before the second house was completed and had not issued the certificates of completion. the two houses were worth £700,000 on the open market. as a result of kevin's refusal to pay, john was unable to pay his suppliers what he owed them, and in turn, they refused to allow him to have more building materials on account. john stopped building. as a result, kevin entered the site and used john's materials, valued at £100,000, and completed the third house. kevin then engaged norman to build the fourth house, using his (norman's) own materials at an agreed contract price of £600,000. this fourth house was completed and kevin refused to pay norman. advise kevin