The offices will be owned by a company and most likely sub letted out my personal advise not sure if its correct or not but here goes would be to contact the construction company find out the company that will be letting them eg the letting agent. Then tell the letting agent u want to clean communal toilets corridors etc and clean the offices, they will either say we will put you in contact with the firm that owns the flats or if they are property managing the site ask you to submit a quotation i would have thaught, if you keep knocking o their door they start talking eventually lol
Hope that helps thanks
ray