You only need planning permission for the following .
If its going to be used as a room
A velox window situated at the front of the house.
A fixed Staircase
I did mine myself and only cost me about £500,( its a hard and dirty job )
A mask, Goggles and Gloves are essential
Cut out the hole where the hatch is going to be
Take out the old loft insulation.
Fit metal joist brackets to either side of the loft ( wall to wall ), to hold the new floor joists.( 4" x 2" or you could get away with 3" x 2" make sure there is at least an inch gap between these joists and existing Floor ( this is called a floating floor ).
Dont take out any linchpin joists ( i didnt ) you can but will need to have some steel supports made.
lay in your cables needed for any sockets and lighting.
Board all the floor out with loft flooring boards.
Fit loft insulation to the walls of ceiling to floor sides and board over the top with plasterboard ( you can use the rock wool or polystyrene )
you will need a velux window, ( i put in a small one, gives me enough light and didnt take any support away from the roof. )
i papered over the plasterboard with a lining paper before i painted it, and put in a strip of spotlights.
I also made cupboards allong both sides of the loft for storage space.
If you need any help, or want to see any photos m8 give me a shout.
Edit.... i agree 100% with Highlander , i wouldnt go putting weights in there.