Rufus
Whichever I get will also be used after bikes are in a proper garage so ........
My advice is the same as johnr, build it like a "bsh" and you'll only need to build one once! My wooden garage/workshop has been up since 1995 and is still in excellent condition.
150mm concrete block foundation for the walls, laid on a 150mm deep concrete foundation. Backfilled with field stone rubble to 200mm below the top, blinded and tamped to 170mm below the top with Type1 quarry material and then a final 20mm of fine sand tamped to leave a finished surface 150mm below the top of the blocks. Polythene membrane 1,000 guage dpm, then A142 (6mm) steel mesh, supported 50mm above the dpm, then pour with a mix of 1:3:3 (cement, sand, washed gravel) and tamp level to the top of the blocks. Ensure that the dpm comes above the finished floor level and use bitumen fibre board 12mm as an expansion gap around the perimeter of the concrete.
Stud walls built from either 'cls' or treated roughsawn ~75 x 50, with t&g on the outside. It would have been better if I had used 18mm shuttering ply/good one side or OSB board and put a waterproof/breathable membrane at the frameside of my t&g, but I didn't. So, far, with a bi-annual coat of creosote, not the "environmentaly friendly" stuff, I have no rotten boards. The boards finish a minimum of 100m above a gravel backfill surface against the block foundation.
My rough carpentry doors are not the best, made from OSB 18mm sheets, braced with 4" x 1" and covered with t&g to match the walls.
The roof was originally 'Onduline', that lasted for 15 years, but became very warped and was prone to condensation when warm, wet air from the west arrived after a cold spell.
I bit the bullet and spent money on double skinned fibreglass skylight profile sheet and 50mm foam cored, double skinned coated metal profile sheeting. This transformed the place. No condensation and much better temperature control. The benifit of natural daylight cannot be underestimated.
Second-hand plastic guttering that was going to be skipped, along with down pipes, but using new brackets were also a later addition. Job lots of materials can be found on Gumtree or with a visit to your local tip, demolition companies yard and a quiet word with the operators. My last purchase of 3mm aluminium plate was from the scrapdealer!
Before you put anything in the new workshop, paint the concrete floor, again, with a good quality bonding floor paint that uses the moisture in the concrete to cure. I didn't and consequently the floor surface wears, gets dusty and you can "loose" those small parts that go "ping" in the night!
Happy building this summer and make sure it doesn't get so big that you require planning permission etc. Fortunately, I demolished an old wooden structure and re-covered the same footprint, so it is 34' x 16'.
Bill