The 31-year-old man, from London practice Dixon Jones, was carrying out a survey inside the abandoned Compton building in Liverpool - which is set to become part of Grosvenor's £500 million Paradise Street development - when he plummeted 4m into the basement.
A search and rescue team, two fire crews and a paramedic team were called to the accident in School Lane next to the historic Bluecoat Chambers to treat the man ( Dixon Jones architect suffers major fall in Liverpool).
Early reports suggested the architect had sustained serious back injuries. However, the HSE believes the man only suffered a broken rib.
An HSE spokesman said: 'We understand the architect was there to look at the fixtures and fittings with a representative of the current owner before it is sold on.
'We will now carry out interviews with a number of people - including the architect - an investigation which could last anything from weeks to months.'
Meanwhile, Merseyside Police confirmed they were not treating the fall as suspicious.
A spokesman said: 'This incident is being classed as an accident. There is no evidence of foul play.'