Raising the roof won't cost much, only some 2x4s, ply and felt. Gotta be cheaper than digging an 8ft deep hole and filling it with a few cubic meters of concrete plus reinforcing.
What is the sub soil like by you and how deep is the top soil?
Have you considered the cost of removing all this soil? For a decent pit you can stand in and work on both ends of a car your looking at at least 10 cubic meters of dirt.
You can get a 2 post ramp brand new for under £1200 delivered http://www.ebay.co.uk/itm/2-POST-LIFT-CA...Swqu9U2ie4
Second hand they are much less.
What is the sub soil like by you and how deep is the top soil?
Have you considered the cost of removing all this soil? For a decent pit you can stand in and work on both ends of a car your looking at at least 10 cubic meters of dirt.
You can get a 2 post ramp brand new for under £1200 delivered http://www.ebay.co.uk/itm/2-POST-LIFT-CA...Swqu9U2ie4
Second hand they are much less.