WALLS: Extra rooms
The first step in building a wall is the same as with other major building jobs – write to the council. The local building inspector must approve your plans, check that they are construotionally safe and that the divided room meets building regulations for fire safety, light and air. The details below are for a wall with a door at one end; it could be modified to form an alcove for a baby’s cot or a dining area.
The advantage of timber framing walls covered with plasterboard is that it can easily be modified, or later removed completely without much work or mess. The door can go anywhere in the length of the wall; a window can be set over it; the wall can contain a serving hatch. Start by locating the joists in the ceiling above by probing with a bradawl ; when you find a joist,probe some more to locate the centre. If the new wall is to run in the same direction as the joists, then it must be directly beneath one of them; if it is to run across the joists you’ll have to locate all of the joists (they’re usually 16in, to 18in, apart), but the new wall can be anywhere along them. Mark where the top of the wall is to go. The frame in the picture, is made of 4in by 2in (100mm by 50mm) timber, the 4in making the thickness of the wall, joined with nails and screws.
you can read more here http://robuild.co.uk/builders/index.php/building-a-partition-wall/