Transition periods
A transition period is a negotiated number of years during which EU obligations do not apply.
The EU does not like giving permanent opt-outs (derogations) from the EU Acquis, but prefers transition periods.
Only Malta has a permanent opt-out (derogation) - for the sale and purchase of second homes by foreigners. It is allowed to demand 5 years of residence for foreigners before they may buy second homes.
Links
See derogations.