sorry if this sounds a bit thick but would it be possible to..

[cheeryface]

keep hens in my backyard ??

.

or do i need a grassy garden ?

Think there's a hen section on Mumsnet but I'll try to help.
Hens would prefer grass but if you gave them sandpits for dustbaths and scratching and grow grass for them in a shallow tray or similar I guess they would be grand. I'm not a fan of the Eglu runs and houses (as an example, I think they're way too small unless you let the hens out as well so if your yard was as small as that, say, then no - it wouldn't be ideal.

Also hens are messy, they poo, scratch, are noisy and will destroy all other plants, especially in a confined space. What kind of size are you thinking of?