Change The grid from Meters to Feet

or a Metric Tape-measure

QFT. OP, if you pay attention to Alazarin's post, you will know things it takes many builders quite a while to "get".
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If the dimensions are given in feet and decimal feet, as opposed to feet and inches, you could build it oversized, pretending meters are feet, then when done, scale it down. I think there is a builder's aid that you can use to scale a whole build down. Of course, learning to use the builder's aid might be more trouble than scaling the dimensions down.

If your plan is an scanned image, you could put into an appropriate drawing or cad program, scale it to life size, and use the measuring tools in the program to measure things. If your plan is sufficiently high quality, you might be able to vectorize it, make the vectorized version life size, and then set the units in your drawing program to metric, and if you've picked the right drawing program, selecting a vector would give you the length.

I thought buildings and other objects had been designed in metric for years and years, does anyone still use feet and inches in technical drawing? (I'm British, maybe it's different in USA)

I agree that for the purposes of SL, 3 feet to the metre is near enough. You could build the house to that ratio and enlarge or reduce it as necessary as a linked object(s) when it's finished.

I've just realised I've repeated what's in the above post, in a slightly different way.
Sorry!

I'm not a builder or an architect, but every house plan for a house in the U.S. and every blueprint I've ever seen in real life have used feet.

It's possible that this has to do with how building codes are written.

SL should just switch to using Smoots as the measurement system. That way, SL won't be taking sides in the US (formerly British) vs metric debate.

I don't know about building codes, but it certainly has something to do with standard lumber sizing.

Another approach is to get an architect's scale with a suitable "scale feet to meters" graduation and scan the floor plan and scale it to a suitable size, then measure with the architects scale.

Another approach, if you don't have to have that one particular floor plan, is to get on the internet and find a metric house plan .