How much should I set aside as a contingency budget

Most experts recommend setting aside 5% to 10% of your budget for run-of-the-mill projects. But because there may be hidden issues in your basement—mold, mildew and structural problems are common—it might make sense to have a larger contingency. If you're working with a contractor, they should know how much you may need for the project you're taking on.

Is it possible to finish a basement by myself?

It depends on the scope of your project. If you aren't adding any new electrical systems, plumbing or making large structural changes, then finishing your basement could be relatively simple to do yourself. But if you plan on undergoing big changes, it's probably best to hire professionals to avoid injuring yourself or installing unsafe features.