Emily, with being a renter, I'd definitely check Pinterest for temporary solutions. I've seen DIYs where you can starch fabric to walls with an iron. Also try temporary wall paper/decals, but I'd test a small area first. Cabinet makers or even yourself would be able to create a flush vanity without attaching it to the wall. Plywood, especially hoop pine, has received a lot of traction. It really gives a clean, organic look that you could have custom cut to fit in the area. You could also use it on the wall – it could be stained, left natural, or painted for a feature. Have a look at EchoPanel. It is a decorative, acoustic pinboard that would add a splash of color/design. If it is above your vanity, you could pin pictures, inspirations, etc.

I love the option to work in a private office while still feeling connected to the larger open plan space. And who doesn't want a huge blackboard to creatively brainstorm?

it's underneath our apartment anything that's not used goes in there Christmas tree decorations fold up chairs/trestles for parties DIY tools luggages unopened boxes from when we moved ..invoices ..stuff we have to keep for the business for tax old paint cans etc etc I'd love to clean it out and just have the necessities ..it also houses all the ducts and pipes of the apartment that we have to crawl under ..vacuum cleaner..

anything is possible with enough money but with such a modest budget you might achieve the new bathroom and kitchen OR the extension but it would be difficult to achieve both at the same time but you haven't answered the questions??? what is the distance from the house to the back fence, what are all the floors, walls and roof built out of (ie inside and outside walls, concrete slab on ground, or is it a timber floor with crawl space underneath the house?? and NB where is north? ...what is the shape of the roof (hip or gable tiled roof or??) ...do you have an aerial photo of the roof like Google earth??...extending over the retaining wall shouldn't be a problem with proper cosntruction and there isn't anywhere else to go without losing the best outdoor area that has easy access to inside...the extra lengths of pipe isn't the biggest cost of new plumbing and a ventilating skylight will make a great light source for an internal bathroom...