Make it as big as possible.
Figure out sun/shade patterns before building.
Make sure his hide is on high ground.
Plan it so you can put a faucet near by with a long hose or two.
Plan to run electricity to it.
Plan how to predator proof it.
I used this link below as a “rough guide” but ended up making mine 4’x8’ due to current space limitations and needs. I added locks for security and a covered shaded area to keep the hide area moist. Being in a relatively hot and very humid climate made it a no brainer for an outdoor enclosure but I had to make sure to make it critter proof as the possums and raccoons are brutal around here.
Building an outdoor enclosure for your turtle will take some time and planning. Once your enclosure is finished, your turtles will have a whole new area to explore and enjoy. An outdoor enclosure mimics a turtle's environment more...