There are often practical limitations to measure the airtightness of a multifamily building as a whole as described in EN 13829. The building may be too large; the floors may not be connected with an internal airflow path; or there may be large leaks in the stairway. In such cases, the measurement is performed on a sample of apartments for compliance check purposes, which raises a number of questions especially as legal disputes may arise. Therefore, our objective was to evaluate the limitations of several sampling methods and suggest improvements based on a field data from ten new multi-family buildings, representing 208 units. In each building, we measured a) the air tightness on the whole building, b) in each apartment, and c) in the common areas of the building. The envelope area was found to be the most reliable parameter as a selection criterion for the sampling method, i.e., it is the best parameter we found to correlate airtightness with. Our analysis also confirms that the leakage in the common areas can have a significant impact on the air permeability of the whole building, especially in the presence of with lift shaft and/or basement parking.