Hello, and thanks for your question!
Question #1, The things to be concerned with would be things like How stable is the ground and how large am I going to build this loft. If this is going to hold any kind of weight such as BBQ with people on it say 20 feet by 20 feet. Thats going to be allot of weight that will need to be supported. If its going to small and just hold a few people then you dont need to build it real heavy.
If the ground is soft, then your going to need to dig oversized footings to help it remain stable in the soft soil. You said its going to be in a low area, that means it is probably more likely to flood or retain a higher water content after and during rains. Thats will make the soil more unstable unless you on a good layer of bedrock or hard pack dirt.
If this is going to be 3feet or more above the ground you will need saftey rails and stairs/steps. You want to use treated lumber so that it will not rot away from bugs and exposure to the eliments. When you decide how big you want it you can contact me and I can help you with the size lumber you need.
Question#2 - The owner of a property is aloud by law to improve their property on their own. If you decide to build alarge deck or one that will be complicated yopu may want to hire a contractor to come by and just help you out. If it is going to carry live load (people on the deck) then you will probably need a building permit and inspections. This should be done anyway if your doing it yourself because you dont want to miss anything that will cause injury later on.
I hope this helps answer some of your questions, if so please click the green accept button, if not im still here to assist you.