A basement is the cheapest square footage you can add to a house, because the structure and the roof are already there. It is also the easiest space to finish incorrectly, in ways that do not show up until an appraiser or a buyer's inspector walks through it.
Start with water, because everything else is wasted if you skip it. Before any framing, we look at the grade outside, the gutters and where they discharge, the condition of the slab, and whether the sump is working and has a backup. A basement that takes on water once a year will destroy insulation, drywall, and flooring on a schedule. Fixing drainage first is not an upsell, it is the foundation of the whole project.
Then the code items. Any basement bedroom needs a compliant means of egress, which in practice means either a walkout or an egress window well sized so a person can actually get out and a firefighter can get in. Ceiling height has a minimum, and it is measured to the lowest obstruction, which is usually a duct or a beam rather than the joists. Mechanical rooms need proper fire separation. Smoke and carbon monoxide detection has to be extended to the new space and interconnected.
The things that make a basement feel finished rather than merely covered are mostly decisions made early. Framing the walls slightly off the concrete with rigid insulation behind them keeps the assembly dry and warm. Running the low-profile ductwork before the ceiling layout is set keeps the height. Putting the recessed lighting on a plan rather than centering it in whatever bay is available is the difference between a room and a basement with lights in it.
Finally, permit it. An unpermitted finished basement does not count as living space in an appraisal, and it is one of the most common things flagged during a home sale in Mercer and Bucks County. Doing it on paper costs very little compared to explaining it later.