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.

the 9' ceiling height will definitely help you with this. Here's what I would suggest (not knowing how far the side window is from the wall with the door on it: Use the wall with the door as the bed and closet/storage area. Build a closet, and put a loft bed at the top. Because it would have to be custom built, you can decide how high it is or how much storage you need. Kind of like this: [houzz=] or this: [houzz=] Can the space below the back window become a desk? And or/build a desk there with storage flanking it. Keep in mind that this isn't a very large room, and he'll probably want as much floor space as possible (at least my 8 year old does!).

Maybe incorporate an L-shaped built-in? However, Is it possible to move the wardrobe to a different wall? Because then the existing 4x4 closet space can be used as a sitting area.

Dear Krishna, Houzz is a promising platform full of many talented people. But to get any response for your query, you must provide them the proper information like- What you really want Built-up area Scope of the work like flooring, ceiling, furniture, wall paints etc. Floor plans if available Pictures of site if possible Only then professionals would be able to respond. Also you can check out my profile. Hope this helps you a bit. :)