My piggies have my DC old playhouse (with a few alterations made by DH) .
Its around 6' 6" x 6'6" and I can just stand full height in the middle.
We have three (boar on one side , 2 sows on the other) we have had as many as 5 , according to animal: space calculations we could fit 12 bu I don;t see it.
We have a divide and two hutches (one rabbit hutch , one plastic trunk) with doors cut to allow constant access to their floorspace.- they only go out when we let them, not like rabbits who like to decide for themselves.
It works really nicely. We have electricity from the garage for a light (essential 
) a heater, a fan .
I can remove 2 of the windows (they secure in with bolts) inside is lined with bars+chicken wire+mesh to protect them.
We have a clear plastic screen outside so even if it looks like drizzle , they can have air but be protected .
Wooden houses DO get hot. Rabbits can climb up onto levels (piggies are lazy) and be nearer the windows.. You can attach a RunAround tunnel via a catdoor or cut a 'door' to their run.
If I could (and I might ask DH to do this for summer) , I'd have our door divided like a stable door and a permenant screening (like the windows) on the upper bit so I could give them more ventilation but I'd need to duck under it (at my age ) .
I've tried a screen door but it didn't stay in place well and my pigs are too precious to risk.
Look on sites like Gumtree , you might get a small shed rather than a playhouse then customise it.
I've put a load of wood/board down this year (on the wooden base floor) and treated the base again with wood protector.
Look on sites like Pinterest or google rabbit houses/sheds/playhouses. Good Luck.