I'm currently deep into shed building territory; 17'x9', built from Travis Perkins and D G Heath (Pontardulais) best timber stock. Lucky my brother in law is an experienced carpenter and lucky he likes doing this sort of stuff because he has practically taken over the construction side, leaving me with the groundworks and passing up any tools he drops.
We have used real railway sleepers to form a perimeter foundation, filled the space in the middle with several tons of road scalpings, and made a floor from 6"x2" CLS joists sheeted with 3/4" exterior grade ply. Walls are 4"x2" CLS at 14"/16" centres. Apex roof from same material, sheeted with exterior grade plywood. Eventually the roof will have 2 layers of bitumen roofing roll and the sides will be done in shiplap, insulated with Kingspan panels inside and lined with plywood. Then we'll get to grips with fitting doors, windows, security, electrics and internal fittings. So far the projected cost is in the £2k region, vs. many times more than this for something less bomb-proof.
I did say to the good Mrs. P that a garden railway would look nice running around our house, and that was the reason for the shed dimensions. She didn't discount the idea, whereas she has totally put her foot down against me filling it with vintage motorcycles (although there will be quite a few old bicycles in there once it's done).
2011's project by way of appeasement will be a greenhouse, but that should be a piece of p!ss compared to my gentlemen's refuge.
Should we perhaps have a "shedhead" category in the Manband Forum?
Oh, and the shed will have some nice big speakers in it so I can play Man tracks (there - made it - a reference to Man and sheds)