Garden Office Build Sense Check

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Hellooo,

I’m planning on DIY’ing a 3m x 2m garden office later this month on an old concrete garage slab. Before I buy the materials and fully commit, I’d appreciate a final sense check to ensure I haven’t missed anything glaringly obvious or over/under complicating things!

Here is the current plan:

Base & Damp Proofing

Slab: Utilizing an existing concrete garage base.

Perimeter: 3m x 2m footprint laid with engineering bricks.

DPM: Heavy-duty Visqueen damp proof membrane over the slab.

DPC: Damp proof course roll running over the engineering bricks directly beneath the sole plate.

Walls & Sheathing

Framing: C16 4x2 stud work, built directly on top of the engineering bricks.

Fixings: M10 threaded rods resin-anchored through the sole plate and bricks directly into the concrete slab.

Top Plate: Double top plates, overlapped at the corners to tie the structure together.

Sheathing: 11mm OSB3. (Question: Should I butt these sheets tight together, or leave a 3mm expansion gap? Finding conflicting info online).

Wrap: breathable membrane over the OSB.

Roof

Joists: C24 treated 6x2 timber at 400mm centers, running the 3m length haven't got enough overhang space going the 2.5m side including gutters. Fixed with nails then joist hangers. Of course noggins etc. Firring strips fixed to the joists for the slope.

Decking: 18mm or 22mm t&g OSB3. Given size I'm thinking 18mm is plenty sufficient?

Covering: EPDM rubber roof system.

Cladding

Front and 1 side (Accessible): Timber shiplap cladding over a 25mm battened airflow cavity.

Back & 1 side (Boundary Access): Metal box profile sheeting (doing this because boundary access will be incredibly tight once the building is raised and the side will be getting a shed butted against). Planning to pre-clad these walls on the ground before lifting them into position.

Insulation & Internal

Floor: Subfloor DPM, then PIR insulation boards (Celotex/Kingspan/other alternative). (Question: Do I need a timber joist frame here, or can I create a floating floor by laying 18mm T&G directly on top of the PIR boards?)

Walls: insulation rolls friction-fitted/stapled between studs.

Roof: PIR insulation between the joists keeping at least 50mm ventilation gap between roof.

Internal Vapor Control: polythene sheet as a Vapour Layer then plasterboard.

Electrics

Won't be touching this, will let a professional, non DIYer sort this given it will need a supply from the mains.

Anything jump out at you as an issue? Especially curious about the OSB racking spacing and whether the floating floor layout is the way to go on top of an old slab or whether laying joists are really needed.

Appreciate any feedback and If any interest I'll share some photos!