I have built many homes from floor plans and use the rule of thumb 1 tile = 3 ft. like mentioned above by other Simmers. Except for very, very simple plans, I have always still had to improvise to make the floor plan work and be realistic in the game. For instance, a lot of times I don't include pantries or small hall, bedroom and utility closets depicted in the plans. If it's a large bedroom closet, I'll sometimes use those we got in GT, but not always. I usually just make some rooms a bit bigger.
I've never used grid paper to map things out, but I think that's a great idea! I just build the outside shell first, place windows/doors, the roof, then work on getting the interior room walls set up as best I can and place items as I go along to make sure my measurements will work, tweaking as needed. Once that's accomplished, then I paint all the walls inside and out, add wall decorations (using MOO if needed), and finally, landscape.
I've found so many cute houses I want to build but end up not doing them for one reason or other like; curved walls (as mentioned above), fancy windows that don't have a comparable version in the game, different levels of stairs leading up to the house and split levels. Hopefully with the new terrain manipulation tools we'll finally be able to do split levels. I'm hoping so anyway. I want to build my in-laws old house, but haven't been able to do it since it's a split level and has a sloped side yard and also a walk-out basement.
It can be such a challenge building homes from floor plans in TS4, but when it works out, it's so satisfying! Sometimes it's a lot easier too than just trying to build something on the fly from one of my own hair-brained ideas. lol Although, I tend to build on the fly more often than from floor plans and it usually works out fine. Just takes longer since I don't have a real guide other than my own sense of creativity. ;)