When using houseplans, what is a good measure to use?
Example: 1 sims square = 2 square feet

When I build lots based on real life floor plans, I eyeball it based off of where the windows, doors, and other prominent features are. If you get those roughly in the right places, the rest will fall into place.

I also make sure to keep sim-limitations in mind, such as a double bed needs to be in a room at least 4 squares wide, regardless of what the original floor plan shows. Same thing for the area around door tiles and parking spaces.

The canonical size of a sim square is a square meter.

Well when I builded my house it came out perfectly, and all I did was use one square as one tile.

I use half the real measurement in squares. So if your real plan calls for 12 feet, do 6 squares, if that makes sense.

I actually figure that a square is 2.5 feet. This has to be tweaked to accomondate certain optimal room sizes. For example a room with a double bed, at a minimum has to be 4x4, larger if you want additional things like dressers. The optimum bathroom, IMHO, should be 3x2. You have to strike a balance between comfort and cost -- that's the real challenge.

According to the actual code, a square is 1 meter, or 3.28 feet. This is the canonical measurement and requires no "figuring".

1 meter is good for a rough measurement. However, as purplebookworm2 mentioned, what really is much more important is door and window placement... If you have a single door centered on a section of wall with two small windows on either side, even if the house plan says that's only 6 feet wide (or whatever) you'd really better make it 3 tiles wide (about 9 feet) so you can fit the single door, plus a window on each side. Same with navigation - sims do much better, especially in a house made for more than one or two sims, with a hallway that's at least 2 tiles wide. In the plans, the main hallway may only be 3 or 4 feet wide, but it's better to widen it a bit to about 6 feet (2 tiles) wide, to make it easier for navigation. So... roughly 3 feet/1 meter, but fiddle a little to make it actually playable.

I have to agree with purplebookworm. You just pretty much have to eyeball it. I tend to automatically know what doors/windows I'll be using on a particular house the moment I decide on a floor plan. Therefore I already know how many tiles I'll need for the doorway. The same goes for windows.

Plus I have general measurements for rooms, for example:
Master/Double Bedroom is 5x6, Single/Child/Nursery is 4x5 or 5x5 depending on whether you expect another single bed to eventually go in that room. The 5x5 measurement allows for two single beds, up to 3 nightstands and a dresser, depending on door placement. Hallways are ALWAYS 2 tiles wide for me as I hate to see my Sims stop and tap their foot with frustration because they can't get through when another Sim is walking down the hall. This always fixes or avoids the stair issues a lot of people (including me) have had because you will leave two tiles in front and behind the stairs at the top. I make small living rooms at 5x5 generally but have been experimenting with larger living rooms such as 5x7 or 6x7 to accomodate desks as well as the living room furniture. I always leave 2 tiles for porch or decks so I don't have issues with doors and stairs as well. And finally, my small bathrooms are 3x2 and larger/master bathrooms are 3x4, you'll see that these measurements also work well with the hallway sizes so that you don't have odd shapes or dead space.

With these measurements I'm able to build houses almost exactly to scale and pretty much identical to what's shown on the plans (if not nicer!). I hope this helps, I know building from plans can sometimes be confusing. But if you become comfortable with your designated measurements and pre-plan, building can be super fast and tons of fun!!

The trouble with a square meter is your rooms, as a general rule will be too small. I look at the typical bedroom in many plans, and they are around 10 feet by 10 feet. If you adopt the meter square metric, this comes to bedrooms of 3x 3. This simply is not practical in game term.

If you go with 2.5 feet, you get a small, but workable 4 x 4 room. But even this is really tiny. You will fit in little more than a single bed and dresser In most real houses, bedrooms will accomodate a double bed and dresser, so even a size of 2.5 feet square is rather small.

2.5 feet is smaller than a meter which is 3.9 feet (39 inches), so I don't see how you can get a larger room by using the 2.5 feet method rather than the metre one.

For some reason I can't think tright now but I'm sure I use two metres per square for big houses from house plans. Don't quote me on it, my brain is numb right now with CAW.

I agree with the above comments that say your judgement is right, I also use the window and door method. If you need a one square window then why have two squares in that space, etc.

well that throws it off right away. A metre is 3.9 feet, 39 inches. A yard is 36 inches. Old dressmaker here haha. So where does that leave us with the no figuring thing?

(39.37 inches/meter) x (1 foot/12 inches) = 3.28 feet/meter

I usually can't use house plans "as is" in the Sims 3. I always have to adjust plans to account for pathing around furniture, doors, staircase landings, and through corridors.

I use the cake test to check out scale in kitchens and dining rooms: Throw a birthday party. If everybody can get a piece of cake, you're good.

I just build the half of the size when it´s possible For exapmle: 16ft - 8 squares. If there´s a 3ft or 15ft wall just make it the next bigger number: 3ft to 4ft and 15ft to 16ft.

Me too, I just half it two to one, works for me. I also work in feet rather than metres. So if the plan says 7 feet, like you say, I'd make it eight squares