Ah yes, not too much to go on in you drawing. Here is what you could do to estimate a long line in the drawing so it scales correctly in SH3D.
- Assume a rectangle top wall and right wall (incl lift shaft), shape is not the same, but area looks roughtly like the size of the flat.
- Imaginary overlay rectangle is 238×390 pixels, a ratio of 0,61:1.
- So the right wall from top of lift shaft to corner is roughly the square root of area/0,61 = 41.8 ft.
- In SH3D, import drawing and set the measurement along the right wall (including lift shaft) to 41.8 ft (≈ 41'10").
- Draw a room inside the flat and see what SH3D reports the area to be (advanced editing plugin is useful to scale room sketch).
- If area is too small, increase the length in the background image (or vice versa).
- Measure different distances in the flat and see if they make sense, and adjust again if necessary. As hansmex said, look for standard measures such as door width, kitchen top width, tub width etc.
- Once you have the background plan (fairly) correct, delete all the test sketches an start drawing.

Sent you a quick test by mail not really knowing the floor area calculation (different in different countries), but I assumed the green area was supposed to be 1066 sqft. Result (or original) not sufficiently accurate for millimetre construction or official purposes. But test indicates that the guesstimate is reasonably close, probably as close as possible given the original.

Good luck.

ok

PS. Just noticed that hansmex has posted a very similar file. My assumption was that all of the green area was 1066 sqft (see above), but I may be wrong.