I am several years in to building a loft layout. Fairly modern (1970) three-storey terraced townhouse in London. In summary, I have found:
1) Make sure the joists are up to it. Get advice if you are not sure. Do NOT cut into roof support carpentry!
2) Get someone who knows what they are doing plumb in plenty of lighting and electric power points; you cannot have too many sockets. Think how many you need, then double that ...
3) Then add more lights. You cannot have too much lighting.
4) Get a sturdy ladder. Do not rely on a step-ladder or some ad hoc arrangement with something balanced on the aperture edge. At the very least invest £30 or so in a proper sliding affair. They are easy to install.
5) Flooring: I used thick, floor-grade tongue-and-groove 8'x4' sheets of chipboard. Do NOT rely on the packs of 'loft floor' stuff they sell in DIY warehouses. It is basically shredded wheat ... if you can afford it use marine ply.
6) Make sure you have not drilled or screwed into any wiring by mistake. Or water pipes ...
7) Think seriously about insulation. The main problem is heat in summer. Before you start building your layout install thick insulation board between the eaves. This REALLY makes a difference. I reckon I have cut midsummer temperatures by 10C as a result. (And you house fuel bills will go down).
8) Dust & crap: hoover, hoover then hoover again. You would be amazed how much dust the average attic accumulates.
9) Now put some more lights in.
10) Needless to say, I did not do any of this, or rather I did, but all in the wrong order. It's OK now, but I'd have saved myself a lot of bother if I had thought a little before leaping.
Hope this helps