Swift:

Your question isn't very clear...

If you want to use a drawing of a floor plan as a guide for your design, it is not imperative that you have the exact dimensions: you can use your best estimate. In case you use an American or English floor plan, chances are that dimensions are rounded to multiples of feet, and often there is some form of repeating pattern in room dimensions.

However, in case you discover the true dimensions at a later moment, you need to remember that you cannot scale your finished drawing at a later stage to reflect the true dimensions.

Hans