I believe you only need planning permission if any work done isn't structural, and if the room doesn't have a bed in it, then that makes a difference too. At least that's what the chap in the 'Loft Shop' told me.
Most loft conversions involve fitting steal girders to hang the new floor on and support the roof - they cut out the purlins and the supports so the roof need something strong to support the new wood that'll be holding it up. The old supports go from the roof to the central load bearing wall.
You may need to have two layers of plasterboard on the bedroom ceilings for fire protection, or some wire hangers to hold the rockwool insulation in place in case fire gets through the bedroom ceilings.
All downstairs doors have to have closures fitted with 30minute fire doors to delay fire reaching the roof. A means of escape window has to be fitted at the front of the house so the fire brigade can rescue you if need be.
When you fit insulation in the roof, you have to leave an air gap so that air can still circulate behind the insulation (between insulation and roof felt). You then fit a plastic vapour barrier before fitting the plasterboard. Two layers are better for sound insulation, but add to the roof weight.
Stairs must have a 2m headroom, and must conform to minimun tread height etc IIRC. I can't remember at what point a loft ladder isn't considered adequate.
If you want to strengthen your existing loft floor, one way to do it is to double up on the existing wooden beams - it's called 'sistering'. Your house will probably have 6 x 2s at 14inch centers, and the loft floor will only be 4 x 2s. Fitting another set of 4 x 2 alongside each and every existing one, and screwing them together will add the extra strength you need. You will need to ensure you fill it all with rockwool for fire prevention before fitting the flooring. The local regs may want the extra plasterboard or the netting if that applies.
Don't forget that any existing wiring will be affected by any insualtion it touches due to heat retention - it effectively reduces it's current carrying capacity by half, so if it's fuly enclosed in the insualtion, you may need to increase it from 1mm to 1.5mm cable (or more if it's a long run) IIRC.
There's probably lots more stuff, but that's all I can remember of the top of my head right now..
Gary.