AHMM co-founder Paul Monaghan said that the Old Street building reeked of a 'Victorian sweatshop' before his practice got to work.
Monaghan was responding to accusations made by international affairs magazine Diplo. The magazine claimed that in 2004 Shelter spent £200,000 more on the refurbishment process of the new headquarters building than it paid out helping to accommodate homeless people.
The architect, whose renovation programme included refurbishing lifts, revamping a reception area which he claims originally looked like 'a delivery bay' and installing new electrics and heating, continued by describing the original structure as 'completely dilapidated'.
He said: 'You could easily spend £5 million on a building like that. The amount we spent was nothing. I suppose it's a moral dilemma for a charity - how much do you spend on your headquarters building, but I wouldn't have let my people work in those conditions.'
A spokesperson for Shelter said: 'Charity regulations state we have to keep some money in reserve, which cannot be spent on frontline services. Rather than leaving this sitting in a bank account, Shelter decided to invest this money in our headquarters, in effect transferring the money from cash reserves to property reserves, in order to help secure our financial stability for the long term.
'Raising funds inevitably requires a small initial investment but last year Shelter, by spending £6 million, raised £21 million in donations.
'Finally, the money Shelter gives out in housing aid grants to other organisations is a very small part of our overall activity to help those in housing need - last year we spent £24 million on frontline services and campaigning to change policy and legislation.'