ICAO Annex 11:
2.10.3.2 A lower limit of a control area shall be established at a height above the ground or water of not less than
200 m (700 ft).
2.10.3.2.1 Recommendation.— The lower limit of a control area should, when practicable and desirable in order to
allow freedom of action for VFR flights below the control area, be established at a greater height than the minimum
specified in 2.10.3.2.
2.10.3.2.2 Recommendation.— When the lower limit of a control area is above 900 m (3 000 ft) MSL it should
coincide with a VFR cruising level of the tables in Appendix 3 to Annex 2.
Note.— This implies that the selected VFR cruising level be such that expected local atmospheric pressure variations do
not result in a lowering of this limit to a height of less than 200 m (700 ft) above ground or water.