Hi GM
Well, Do you know how long it will take?
You say communal areas so you have more than one area. So is it in more than one building?
Generally work out the time it take, material, all over heads, petrol etc and then add on a percentage that you would benefit from within reason.
So the more hours, you can give slight less hourly rate, if its only 1 hour then the price goes up!
Dave