No idea what your shed looks like or the size. A simple sand floor would be easy to clean and maintain. Maybe add a skirt on the outside horizontal to the ground to keep animals from burrowing in under the walls. Plenty of ventilation, either vents or windows covered with wire mesh. Common window screen or even chicken wire will not keep a determined predator out. Many install a light although it's not really a necessity. Electricity is also optional, helps if heat is necessary to keep their water from freezing. Insulation is not necessary as most chickens are hardy enough to withstand the cold. Some say draft free but actually more out of direct wind unless it gets really cold. Plenty of roosts and that's about it.
Im in the UK so extreme weather is not an issue, the shed is on a concrete plinth so also digging under is not a problem, I have 5 girls all around 18 months old, I think the shed is 4ft by 6ft the coop they have at the moment is tiny lol, they free range from dawn to dusk, what sort of height am I looking at for roosting? Also should nest boxes be off the floor? If so how far?
Nest boxes need to be lower than the roost to prevent them wanting to sleep in them. Just above the floor would work but could be a little higher if you want.The roost height really doesn't matter much. They like to sleep as high as possible. Too high and they might hurt themselves jumping down. A meter and a half would be fine. Might want to put one or two in a stairstep to make it easier for them to get to the top one.
I covnerted my shed to a coop last year. I added a lot of windows, and some nest box (just screwed milk crates to wall) and a 2X4 roost.. I use straw for the bedding.. works great, and is cost effective. shavings cost an arm and a leg to use. straw is around $2.50 a bale.. so it keeps cost down.