Let's say the first on in in A1
Insert a new column B
In B1 enter =--LEFT(A1,LEN(A1)-3); this will retunr the number 5 (if you
have 5 m² in A1)
Copy down the column and add
Now you have real numbers that can be added
For a permanent solution: after you have done the above
Select all the new B values and give them the custom format: 0 "m²"
Select all the B values and Copy, with them still selected use Edit | Paste
Special -> Values to turn the formulas to numbers
Now you have real numbers that can be added
Now you can delete column A
best wishes