Hi all Please excuse me, but I am not a builder, so not sure if I will use correct wording... I would like to hear your views on the best method of creating a suitable base for a summerhouse as well as some other questions I need help with. The summerhouse is to be used as a gym and storage (will be divided by a wall). It will be located at the back of my garden, which is about 35m from the street. The plan is to insulate it, so we can use it in winter, with (fan) heater installed (there is an existing power supply to a shed which will be demolished). I am not sure how long we will live in the house, so potentailly I would like to add value by this addition. Outer dimensions will be approximately 6m (wide) x 5m (deep), gym 4m x 5m, storage 2m x 5m. I have decided to build the floor from 2x4 treated timber (I am aware of the span and spacing requirements) and use the same for all of the walls and pitched roof (low pitch). I would like to insulate the floor withn 50mm celotex sealed with aluminium tape, supported by battens (so that insulation sits flush with top of the joists). On top of that ply or osb? Can I use rockwool slab insulation instead of? If I can, what would be other pros/cons and requirements? How will this affect teh base design? Walls structure plan (from external): cladding (which is the least maintenance and will not fade over time?), breather membrane, insulation (celotex or rockwool?), internally boarded with osb. Did I miss something? I have not yet decided about the roof structure yet. Any suggestions are welcomed... I would like it to be lighweight. Judging by the dimension of the building (5m deep) I suppose I will need a ridge beam as the roof if not flat will be of low pitch. Anyway, the base... There is an existing concrete slab (or rather 6 smaller slabs, adjoining and in a brickwork overlap pattern). The depth is between 2" and 3" and is set on a shallow 1" - 2" compacted hardocre. The base dimensions are 3m x 6m, so is too small anyway. The individual slabs are not level, which makes the whole thing even more awkward. Two of them have a light(?) slope, no more than an 1"/3m. I am considering: 1. Diging up a trench for the foundations where there is no existing concrete base. Following on from that I would lay a course of brick or blockwork around the perimeter of the footprint and where the joist span would require to make it all level; or 2. Use railway sleepers (I can get my hands on about 30 free of charge , which have been stored in a dry warehouse), lay them over existing concrete slab and level the surface as I lay the floor joist. Where there is no slab, dig up about 4" - 6", fill and compact with crushed hardcore (or shall i use shingles?) and put the sleepers down. 3. Break the base, dig up for the full foundations and where the joist span will dictate (I assume this will be the most pricey, due to concrete pum required). Any other suggestions, or points which I should consider?