Floor

You need to give careful thought to the floor.  Get it right first time.  I should have insisted in having a proper flat finish as would for a kitchen floor but what I ended up with is rather agricultural.  This causes two problems it is hard to clean up and not very comfortable to lie on.  I have got a judo mat to lie which works out much cheaper that refinishing the floor.

Pit

If you want a pit do plan for it now.  Much better than smashing up a new floor.

Surface finishing

A properly flat concrete floor can be painted with special paint.  Can also get tiles for garage floor.  

Damp

Make sure that there is a damp proof membrane under the floor otherwise you will get water coming up from under keeping the garage damp. 

Ventilation

Condensation is a problem in lots of garages. Prefab concrete garages can be particularly bad - think about dry lining the walls. Fit air bricks or similar to ensure a through draft.

Roof

It was suggested that I have a metal sheet roof.  It looks like the stuff you see on farm building and consists of large sheets of plastic coated steel.

I was told that a felted roof would not last.

I was also told that tiled roofs take much longer to build.  Generally time is money.  A pitched tiled roof would have rafters which can provide useful storage.  My metal roof has much more head room.

Engine hoist

A strong beam or girder can be fitted with a hoist to lift heavy stuff like engines. Plan where you need it early as this may require stronger walls to support.

Walls

Wood, concrete, brick or metal.