Like me you are really fortunate to be able to start from zero with your listening room. I had been doing hi-fi for 50 years before my chance came.
There are already some great suggestions above, so I won't repeat any of those..
My room is in the basement. As I've said in other posts this gives real advantage by enabling all equipment to be in hard contact with the floor/ground so it cannot move or vibrate, causing distortion of the signal and corruption of the soundstage. Ensure the mass loading is maximised by using heavy support materials.
A basement siting also removes the temptation to include windows. Glass is about the hardest most reflective substance there is and can reflect sound waves behind curtains and blinds. Concert halls and recording studios don't have windows.
With an empty page, acoustic design starts from scratch. So there is every chance to get it exactly right. There is a temptation to over-damp with thick layers of padding on the walls and thick rugs. Even ceiling treatment. This is not the best way to go, some sound reflection is required, to create a realistic soundstage and room boundaries. So I very strongly recommend paying for an acoustician with relevant expertise and experience to advise; the price will be very small relative to the overall construction cost.
Prepare for a shock. You will have a bigger sound quality upgrade than an order of magnitude more expenditure on equipment, wires, tweaks, the lot.
