You could certainly talk to a cabinet maker and have something custom made to your preferences and design style. You could also talk to a closet design specialist who might be able to utilize every square inch of that space more effectively while working with a cabinet maker. Part of how the space is designed will depend on the age of your girls. If they are under 12 or so it shouldn't be too difficult. If they are teenagers, talk to them and find out what kind of space they need.

Since the lot you describe is smaller than my open living area, I would suggest looking at some of the "shotgun" house plans around New Orleans,LA. I think you can build up and back and get an interesting home with a loft for space. It can be done, A porch with an entry to a narrow hall at the back might work best for you. Will you plan a courtyard (closed)? or what? What part of the country are you in? What are your local codes?

How are the plans for the bedroom coming along?

I suggest you follow the step-by-step guide for creating storage cabinets under the bathroom sink. Take proper measurements - Measure the available available space in height, width and height. Cut Plywood to size - With a circular saw and framing square, cut all the required pieces out. You can use a sliding compound saw as an alternative. Attach the base and sides - Start assembling the sink cabinets by placing the bottom piece of plywood sheet on a flat surface. Attach the back and top boards - Place the plywood sheet for the top part of the cabinet and attach wood screws to connect it to both the sides. Turn the back faces of the cabinet upwards. Keep the back piece over the structure and fix it into place with wood screws. Attach shelves - Place the cabinet in an upright position to install the shelves. Slide a shelf into the structure so it is flush against the back. Attach door - Keep both hinges of the door on the same side of the plywood sheet. Drill them using wood screws into place. Now keep the door over the cabinet structure, extend the hinges to one of the side panels and fix them to make them secure. Fix a hasp closure on the other side of the door, placing it in the center of the hinges.