hello! :wave:
I'm no expert, but I think bunnies are generally fully grown for definite by about 5-6 months at the latest. How old are your buns?
I think that the hutch on the paving slabs is fine, my bunny is the same when he is outside. I think as long as they have an 'upstairs' or somewhere they can get off the cold ground if they want to, they should be fine.
my bunny is an indoor bunny tho, so I'm not sure exactly about the favourable outdoor housing set ups.
This thread might be quite helpful to you tho
http://forums.rabbitrehome.org.uk/showthread.php?t=66961
as it has links to pictures of members housing set ups :thumb:
EDIT to add that if you've moved them off the grass, it might be worth keeping an eye on their hay, and making sure they have enough, as they'll probably eat more without unlimited access to grass that they once had.
Also, if you have room, and they like to dig, you could possibly add a carboard box filled with hay/shredded paper for them to dig in? My rocky loves these!!