Four private equity firms are circling Lord Foster’s practice, the second round of bids for which are due in by the end of the month.

A source told Building that the mystery bidders have each been invited to meet this week with Lord Foster, followed by a session with his senior management.

Initial estimates of the firm’s value have reached £500m. Despite speculation that it has been overvalued, a spokesperson for Foster said buyers were willing to pay about that price.

The spokesperson said: “It’s not a pie in the sky figure. It may not go for £500m, but it will get at least £390m. I have no doubt whatsoever that it will sell in that price range or not at all.”

The news comes as it emerged that Foster spent about half a million pounds rebranding the practice. The name changed at the end of last year from Foster and Partners to Foster + Partners.

Observers say the new name distances Foster from the rest of the firm and point out that he has been priming the practice for sale since it made a loss in 2004 by cutting back on overheads and driving turnover up.

Foster’s 2005 results revealed that contributions to the directors’ pension scheme decreased by almost 20% from 2004, despite the fact that there were the same number of directors in the scheme in both years. Also, the firm donated £32,000 to charity compared with £89,000 the year before.

In 2005, the firm’s number of employees increased by 100, yet total wages went up by £3m – an average of £30,000 each.

Catalyst Corporate Finance is handling the sale on behalf of Foster, who is expected to retain a stake in the business.

He has already made moves to reduce his ties with the firm by putting his penthouse at its headquarters in Battersea, London, up for sale.