A friend of mine has recently LEASED an area of common property, unusable by any other owners, for ten or so years. This has allowed her to expand her ground floor terrace into a fenced garden, and she has responsibility for the maintenance of the patch.
I think this is the best way to go as it provides some income to the scheme while retaining a level of control over what the area can be used for and not used for (specified in the lease).
For instance, you could specify that it may be fenced off for a barbecue area but not used as an extra car space.
Just make sure fences etc are the responsibility of the owner and will be removed at the end of the (generous) lease period and it saves you all the hassle of redistributing unit entitlements and all that jazz.
You could even get agreement in principle from the committee. Just set a reasonable fee and everyone will be happy.