Reading House Plans

[LlamaMama]

So a 40x40 lot would equate to 80'x80'?

[samoht04]

I thought the small squares were just for extra placement, I don't think they count them in the size, I think it is just the big squares.

[LlamaMama]

What size would you consider the large squares to be, samoht?

[Aiwe]

I figure that the large squares are about two and a half foot wide as a single door just fits along the side of one square, my (real life) kitchen has six inch square tiles on the floor and five of them take you from one edge of the door frame to the other so the door must be about two and a half feet wide. Hope that helps.

[samoht04]

Well your measurements may be right but the small squares are placement, I think there is an option to turn them off inside a house?

[LlamaMama]

I recently made a house from an online house plan and I did it this way: an 80x80 ft. house was made as 40x40 big sim squares. It worked out nicely.

[mtglady]

I have done some meshing for Sims 2 and Sims 3.  I created different styled chairs, tables etc. for my simmies to enjoy.
The 'footprints' (the green placements) for the furniturealso gave me the measurement basis for my home plan conversions.  I usually plan everything using graph paper with furniture cutouts (like a kid with legos or building blocks)   . 
A twin bed, dresser, full table, desk, bathtub or loveseat takes up 1 square width x 3 squares in length while a full bed or sofa needs 2 squares width x 3 squares in length.  A kitchen cabinet, stove, refrigerator, night stand, chairs, toilet, shower, and sink all need 1 x 1 squares.  These items need a 1 x 1 square of clearance in front of them to function correctly (chairs need the clearance behind them).
A stairway needs at least 6 squares in length to function correctly (this includes the top and the bottom steps) and the width is 1 square for each step wide.    The single doors actually take up only a 1 x 2 sim square area. 
So Telsey is fairly correct in thinking a sim square equals 2.33 feet in true life although some Sim footprints tend to lead one to think they are 2.5 feet square as Aiwe stated.   
Hope I haven't confused you  .