The recommended amount of space varies depending on the size of the bird and whether or not they are cooped/penned all the time or only at night/bad weather (and the amount of time that entails, would of course be important.) According to The Chicken Health Handbook, the amount of space needed for adults is:
Open housing (free ranging available each day) is
4 sq ft per heavy breed bird,
3 sq ft per light breed bird and
2 sq ft per bantam.
Confined housing (no free-ranging) is
10 sq ft per heavy breed bird
7.5 sq ft per light breed bird
5 sq ft per bantam
For larger bantams such as silkies, cochins, brahmas, the light weight measure is more appropriate.
For 18 confined birds you need 135 sq ft (calculated as light breed space) total. You have 240 sq ft. If you have large breeds, you would need 180 sq ft.
Open housing (free ranging available each day) is
4 sq ft per heavy breed bird,
3 sq ft per light breed bird and
2 sq ft per bantam.
Confined housing (no free-ranging) is
10 sq ft per heavy breed bird
7.5 sq ft per light breed bird
5 sq ft per bantam
For larger bantams such as silkies, cochins, brahmas, the light weight measure is more appropriate.
For 18 confined birds you need 135 sq ft (calculated as light breed space) total. You have 240 sq ft. If you have large breeds, you would need 180 sq ft.