If you are using doors at all in your little house, perhaps they will only be the bathroom door or the bedroom door. Privacy is an issue if you have guests. Even the most stalwart will have a hard time dealing with a bathroom without a door, even if you are married.
Well...You might as well make storage out of any door that you can. It is a little haphazard looking to just plunk a set of shelves on the back of a door. You must make it neat and beautifully finished in order to make it look like it belongs there and sleek enough to keep it looking like the fabric of the building rather than a clumsy add on.
Remembering that the thickness of the door frame will impact the swing of a newly thickened door, you must make the storage unit much narrower than the door. The unit will be on the side of the door that does not have the hinge pin. The hinge pin is on the flush side of the door, and the opposite side has the thickness of the wall and the woodwork.
You can really make the unit as deep as you want, but if it is too deep, it will impact the useful space of the room inside.
Decide ahead of time where the shelves will go along the sides of the unit. Pre-cut the dadoes for the shelves, or make a simple saw cut to hold the glass shelves.(have plate glass shelves cut at your glass company, and the front edge only finished and polished). Cut about half way through the sides of the unit boards..
If you are working with a bathroom door, you should measure your usual toiletries or rolled up towels etc., and use the largest as the width of the shelves. Make a simple rectangle to fit the space(remember that the shelving unit must be narrow enough that it does not affect the swing of the door past the jamb. Top and bottom are less important) using butt joints, glue and screws or finish nails. It is unimportant which as they will not show. Use one by whatever width, pine, Poplar or other hardwood. The hardwood will need to be pre-drilled to accept the fasteners.
Mount the rectangle onto the door with construction adhesive and finish nails through the sides, top and bottom at an angle into the door.(from the outside) Hollow core doors will require special toggle hangers and tiny angle irons to secure the unit to the thin skin of the door. These will be hidden by the trim.
Slide in the shelves. Gluing will not be necessary.
Drill through the frame, about 1/3 of the way above each of the future shelves, and 1/4 of an inch from the front edge. Slide in a dowel, brass or stainless steel rod across the entire unit to prevent things from sliding off the shelves as the door is swung. Rubber like matting used for shelves or the rubbery matting for under area carpets can be cut to fit on the shelves to minimize sliding around on the shelves.
Picture frame the shelf unit with layers of molding. Cut out bits of the inside corner of the molding to go around the angle iron if present. Do not glue this molding except at the mitered corners, in case it has to be removed in the future. Attach the frame with finish nails. If the shelves are deeper than any commercial molding available, make sure that the joints of the initial rectangle are invisible from the sides.
Make several layers of molding to get large or deep moldings, filling voids with bits of board. (see the drawings).
When the frame is done and attached, cover the front edge of the unit with half round or astragal molding so that no seam is showing from the front. Miter the corners.
Paint or finish as you like.
If you would like a set of shelves to use in a kitchen, try the following:
I am thinking of a useful unit to serve from(in a modest way) and for storage(also in a modest way).
Mount a heavy half door in your kitchen doorway. I assume that complete privacy is rarely necessary in the kitchen unless there is a trend for clouds of black smoke to emanate from it.
Make the unit at a comfortable height for serving, and the door should swing out from the kitchen.
You can make this plenty deep, again allowing for the swing of the door on the knob side. Make sure you have at least three heavy duty hinges even on this half door, as the weight may be excessive.
The attached shelving unit should be flush with the top edge of the door.
Drop on a counter top of granite or any other composition material. Choose a thinner granite or marble that will be available in limited colors and do not put heavy weights on the counter top. The sides may go right up to the side of the door at the leading edge of the door, and may hang out into the room(again, allowing for the swing of the door on the outside this time, and curving the inside of the counter top to allow for the swing from the inside). Make both sides mirror images despite the fact that the inside hinge side will not affect the swing of the door. At some expense, stone or composition materials can be formed into moldings to epoxy onto the top edge of the counter tops so that things will not slide off the edge of the counter if the door needs to be opened.
An alternative counter top might be mahogany or teak, as they both resist spills if well sealed.