It does have a staircase but as for being classed as another room when the house is sold I am not sure but I dont see why not
The basics are
In the UK, if you have 3 floors, the doors will have to have self closers and fire proof to 30 mins. Existing doors can stay but the 3rd floor new door will have to be a 30mins fire door and self closing.
The height of the conservation window or dorma window has a maximum sill height, this is so you can climb out there to be rescue by the fireman on a ladder if there is a fire.
There has to be a staircase designed to building reg - ie, min width, angles, size of the depth and height of each steps, width of each balustrades.
As to cost, it depends on the existing roof design it comes down to.
Type of roof - what frame is it, if its a V frame then you can pretty much kiss it good bye as you'll have to rip the whole roof down before you can make it habitable. If its a A frame then you have a chance.
If its a A frame then you need to check joist sizes and amount of load it can take and how much reinforcements joist you'll need.
Then you'll need to think about insulation (required U-value) and a window, both of which will apply to building reg and planning, respectively. Without a window it will not be considered habitable and to get a window in you'll need planning. Without building reg it'll not be habitable either and failure to get that will means when you comes to sell it, you won't be able to produce the certificate so the options will be.
1 - Apply retrospectively, risky as building reg officers checks on the build on different stages to check it comes to the requirement standards. And if you fail, you'll have to put it back the way it was, at your own cost.
2 - Sell it as a store room, but you won't get as much for your property since it will be 1 bedroom less. Not to mention the lack of the building reg certificate will put most buyers off.
So, get planning (mostly for the window) and building reg.
.
