Looks like you've got a good floor plan to work with!

Judging by it's long skinny attributes, are you in a terrace house? Depending on where you are located, there can be council restrictions with building up because of street appeal/heritage overlays. Check out your neighbours and see if they have any double story additions, this will indicate whether it needs to be built at the back (not visible from the street).

Sounds like you've got a pretty good idea of the direction you want to head, so I would engage a builder(s) to quote on the works. Because it's a tight space, if you want to get savvy with smart use of space and functional layouts, you may want to invest in a designer or architect so you get it right the first time.

Goodluck!!

Hi Bec,

It sounds like you have some great ideas. I would suggest an Interior Designer as they can help you and your family visualize and collate all ideas together in a 3D perspective view.
3D visuals are very important for the client to really see and experience the end result. This way it is easy to adjust the design and tweek it to the exact experience of the end user and set-out the correct guidelines to pass onto the builder.