storage, project/office space and also as a guest bedroom. The bed is a mattress on the floor, tucked away under the eaves (the space was constructed to fit a double bed perfectly) and it has its own low level lighting. It's a really popular place with guests, especially children as it feels a bit like camping and it also means that there's not a barely-used double bed taking up most of the main floor space.
Here are some more thoughts/lessons learned:
Blinds - I bought cheapish blackout blinds designed for velux windows and they've been brilliant - just make sure to factor in as part of your budget.
Heating - it would have cost a lot more to extend the plumbing up to the attic so at the builder's suggestion I went for electric wall heaters instead. The loft is so heavily insulated that they are rarely used, even in winter. And when they're switched in, they normally make the room toasty in about 2 minutes!
Insulation - there are regulations about how much insulation you should have (in order to pass building regs). I recall that it was so thick it did shrink the room down a bit. Was a fairly big deal as ceiling height was doable but with no room to spare, and the required thickness of insulation meant the room 'shrunk' down a fair bit from the planned dimensions
Dormer - I don't have one and the main reasons for not going ahead were 1) I couldn't afford it, 2) the neighbours would have made my life very difficult if I had put in a planning application and 3) no one in my street had a dormer and so I assumed they weren't permitted. If I was getting the loft done now I would go for it as a different neighbour has subsequently had one put in, and I've come to realise my neighbours complain about EVERYTHING.
Flooring - my house is full of gappy, stripped floorboards. I wanted to continue what my builder referred politely to 'rustic' flooring in the attic, but without the gappiness/draughtyness so he laid chipboard (or similar) down, and some reclaimed floorboards were screwed down on top and then sanded, stained and waxed. It looks very nice!
Views - if you have a view you want to take advantage of, have a good think about the positioning of the windows, esp if velux. I wanted to have a view I could see while sitting or standing, but got the measurements slightly wrong and they're a little on the high side.
Cupboards - I've got built-in under eaves cupboards on both sides and feel a little bit meh about them. They're really awkward to access and full of stuff that doesn't ever get used, and is nigh-on impossible to get at! Meanwhile, all the stuff we use more regularly which should be in the cupboards (luggage, sports gear etc) is laid out all over the floor.
Builders - a loft conversion is a really invasive project. Before you start, find out how the builders plan to access your loft, ie via scaffolding or from within the house? Mine put scaffolding up but then proceeded to use the front door every single day (still have no idea whatsoever the scaffolding was for, apart from inflaming my neighbours). Think through some worst-case scenarios and establish clearly how they'll be handled. For example, what if someone puts a foot through the ceiling? What if someone accidentally spills wet plaster all over the stair carpet? (hmm can you tell I'm speaking from experience?) This is all standard stuff regarding having the builders in, but a loft conversion can be a pretty epic experience in terms of having your house wrecked!
All in all, I'm pretty happy with my loft - it's a relatively small space but earns its keep by being a multipurpose room. And as it passed all the building regs, it's classed as a 4th bedroom, which impacts the value of the house.
Sorry for unneccessarily long post - hope there's some vaguely useful bits in there somewhere!