Generally you would create walls in a game engine as boxes which you then link together. You could do this (with most of the walls being based on the same simple box and the oddball areas being separate entities) but it really only works if all the walls have the same basic material. But, if your walls have different materials (like something that is spread across a large area or several of your boxes) then you would have to break your texture down by wall and make sure the right one was on the right box. You can make all the walls one object but then you’ve got a texturing headache again, now you have to UV map a huge complex model. I’m not an Arch Viz guy but the first method makes more sense to me as it requires only a few boxes. Also the larger areas (where the walls seem really thick) don’t actually need to be that thick. Using the multiple box method would allow you to build what looks like what you have but is actually made up of the smaller boxes (assuming there’s no need for the client to actually enter that space).