A cabinet maker. Cost? Well, here in the US, I imagine that it could be done for under $1,000 - perhaps a little more depending on how fancy you want it to be, if you are willing to do the finishing work - paint or clear finishes. It's very difficult to estimate the cost of any type of custom work without actually being there - and that's what this is; custom work which is always expensive.

i want to add a window seat in my room and i dont want to pay alot either im from sydney australia so how much would it usually cost and what type of builder are they? cause idk what to google