Building a partition to divide a large area into two smaller ones is quite straightforward to do, using a frame of timber studs. You can clad the wall and plaster it so the new addition looks an integral part of the house. Before you can go ahead, however, you may first need to seek approval from your local authority.

Before you begin to build a partition wall, check with your local authority to make sure that the space you are creating complies with the Regulations.

These state that if a new room is to be ‘habitable’ — a living room, dining room or bedroom (but not a WC, bathroom or kitchen) — it must meet requirements relating to ventilation.

The regulations stipulate that an open space must be available on the outside of the window to provide sufficient ventilation to the room. The openable area of the windows to each room must be not less than a twentieth of the room’s floor area. (To check this, divide the area of the floor by the area of the window’s sash or top vent.) Also, part, if not all, of the top vent must be 1.75m (5ft 9in) above the floor.

Alternative and additional means of ventilation may be provided by a mechanical ventilator direct to the open air. It may be permissible for a fanlight to connect to a vented lobby.

If you plan to partition a large bedroom to make an en-suite shower or WC on an internal wall, natural light will not be required, but ventilation will. Consider the positioning of the new room in relation to the existing plumbing and the means of ventilation.

Bear in mind the size and shape of the rooms in relation to the furniture — for example, should you plan to make a large bedroom into two smaller units, allow sufficient space for the beds to be made without difficulty. You will also need to create a corridor to make the two rooms self-contained.

Timber-framed non-loadbearing walls can be built relatively easily. The frame is usually made from 100 x 50mm (4 x 2in) or 75 x 50mm (3 x 2in) sawn softwood. The partition comprises a head or ceiling plate, which forms the top of the wall and is fixed to the ceiling joists; a matching length, nailed to the them: this may mean altering the overall dimensions of your planned rooms. Locate the floor joist in question and check whether stiffening is required. If so, reinforce it by fixing an additional joist on each side.

If the new partition is to run at right angles to the floor and ceiling joists, it can be fitted at any point. Each joist will share the load and provide a solid fixing.

If the wall is to run parallel with the joists, it must stand directly over one of  floor, which forms the sill, or sole plate; studs which fit between the plates, equally spaced — about 400mm (1ft 4in) centre to centre — and fixed with nails; and short noggings which are nailed between the studs to make the structure rigid. Noggings are required where horizontal joints occur in the panelling.

Remove the skirting and lift the floorboards. Temporarily lay some of the boards to walk on while working. Screw metal joist hangers to the walls at each end, using 50mm (2in) long screws, to support the reinforcing joists flush with the original joist. Cut two reinforcing joists to fit between the hangers. Allow not more than 6mm (Vain) for tolerance.

Use 12mm (1/2in) diameter coachbolts to clamp the joists together. Drill the holes for them slightly larger than their diameter and spaced not more than 900mm (3ft) apart, working from the centre. Place large plain washers under the head and nut.

Alternatively, you can use 75mm (3in) diameter double-sided timber connectors between the meeting faces instead of joist hangers. If you have room, and a drill bit long enough, drill through all three joists while they are held together with cramps. If not, clamp one in place and drill through the two. Remove the reinforcing joist and clamp the other on the opposite side. Drill through it using the hole in the original joist as a guide. Bolt the reinforcing joists together.

Replace the floorboards on which to erect the partition.