Placing a cupboard underneath a staircase is a functional way to make use of existing floor space that probably does not get used often. Though the stairs are slanted you can make the ceiling level or, for a sleek design and a little extra space, you can slant the ceiling at the same angle as the stairs. Once completed the new enclosure should provide extra storage space, which is always useful.
Lay out the size of the cupboard on the floor. The space will consist of four new walls placed underneath a set of stairs that ascends next to a wall. Use a 5 by 10 cm (2 by 4 inch) and a level to get the front wall even with the inside portion of the outside stair stringer. Allow enough vertical space for a 60 by 200 cm (24 by 80 inch) pre-hung cupboard door.
Construct the side wall at the high end of the cupboard with 5 by 10 cm (2 by 4 inch) boards. You will need just one bottom and one top plate. This wall should be put together first and then inserted into place. The bottom plate can be nailed to the floor and while the top can be nailed between the insides of the stair stringers. (This determines the overall width of the wall.)
Construct the low end side wall in the same manner as the high end wall and nail it into place.
Build the rear wall (it will either have a square shape or be cut to fit under the angle of the stairs). Once this wall is in place it can be nailed to the floor, two side walls and inside face of the stringer. Make sure this wall is level before you nail.
Cut and build the ceiling frame exactly to size and slide it into place and then nail it with large nails. Be aware that a slanted ceiling with involve angled cuts, but might look sharp when the closet is done.
Build the front wall piece by piece leaving the manufacturer's recommended opening for the hollow-core door.
Install a light fixture in the centre of the ceiling, a switch box near the door and then have an electrician finish the wiring.
Cover the inside and outside of the closet with 1.2 cm (1/2 inch) plasterboard. Nail the board with 4.3 cm (1 3/4 inch) plasterboard nails.
Apply plasterboard tape and joint compound to the inside corners and fill all nail holes. You will need three applications of the compound with each coat carefully sanded before the next one is applied.
Add a strip of corner bead to the two outside corners. Carefully nail the bead with plasterboard nails and apply three separate applications of joint compound. Make sure each layer is thoroughly dry and sanded before adding the next.
Install the pre-hung hollow core door. Don't forget to use shims on the hinge side to get the door perfectly level.
Cut and nail 2.5 by 10 cm (1 by 4 inch) skirting board trim with bright finish nails to the inside and outside walls. Set the nail heads and fill with wood putty.
Install a 2.5 by 25 cm (1 by 10 inch) wood shelf and 2.5 cm (1 inch) dowel. Each item will need brackets.
Paint the plasterboard with the primer-sealer.
Paint everything except the door with a top coat of paint. Choose a bright colour and use a sealer on the wood trim first.
Let the paint dry.
Make sure all walls are level. Panelling can be used instead of plasterboard. Sliding or folding doors can be used instead of a swinging door. A cupboard under a open set of stairs in the middle of a room would probably look tacky.
Do not attach anything directly to the stairs. Just use the stringers.