The easiest option would be to be willing to relocate (even just outside of the city) and hire an architect to guide you through the process.
The main benefit of buying a house vs renting it, is knowing that you want to stay in the area because you’ll be insulated from the affects of gentrification. So, I’d advise against any drastic move to a very different area that you’re unfamiliar with.
If there’s an empty lot available that you could purchase, that may be a good option if you know the area and like it. Then you could build on it at a later date after having time to plan.
It may sound silly but even building a rough house in Sims may help with some of your creative ideas, layout and planning!