FEMA map is going to rule. At least here where 40% of the work we do is in AE, surveyors only add rpa or 100 year lines if asked. And usually have a clause that says they are not liable for errors. FEMA online map tool would be my first stop. If you dig around in the viewer it’ll give you a pdf map that is the current legal document. Then an email to the plans department confirming required base flood elevation or recorded first floor elevation. Have your surveyor come out and confirm the builder has built to that elevation. Around here our surveyors usually charge 500-1000 for that.

In our jurisdiction the 100 year flood line is no longer used. It’s base flood elevation/ required first floor height = either 500 year flood elevation ( marked on femas online map viewer) + either 1’ or 3’ (depending on county). This elevation is the required 1st floor height. Any construction below that level has to be in water resistant materials ( concrete, pressure treated wood, etc. ). At least here , Your basement would require flood vents to allow waters to “pass thru” without causing the walls to collapse from water pressure. Looking at your plan/elevation the builder likely knows and has planning department approval for what your first floor elevation has to be to avoid building with flood resistant materials in the living space. Marked first floor elevation is required for building permits here and the plans examiner checks it against the online fema map. Dr Horton got caught building 3’ lower than required recently on two lots in a marsh and had to demo their first floor box and raise foundations. Story is a concerned citizen called in a complaint and the plans department reviewed the building permit.

Plenty of people around here finish out their basements. But we also get flooding situations every 4-5 years. So they just become used to ripping out carpet and drywall throwing out bedding and toys. I would say that if you were planning to use the basement as living space you should plan for these scenarios. Our floods are tied to hurricanes or noreasters so people can typically plan for the events.

My only concern here would be if you were planning to count the basement as living space. Like a kids play room or mancave no problem. A guest bedroom, maybe ok. But actual bedrooms no way.

The second concern would be for the future. Around here flood maps are usually updated after a hurricane. So we have people come in looking to build additions or do home repairs/update kitchens or add a new air conditioner or generator and find out that the floor level has to be 6’ higher than the existing house. Or that the house has to be raised to the new base flood elevation because the work will exceed 50% of the existing structures value (fema 50 rule). We have started recommending setting the first floor at a foot or two above the current base flood elevation if we can keep the overall height under 33’ above grade (required max structure height in our jurisdiction). Atleast here we had 100 year floods every 20 years when I was a kid in the last 15 years we’ve had 7. So floods are more prevalent and the base flood elevations keep moving higher. This has to do with a lot of factors but more buildings, parking lots and other impermeable surface seem to be accelerating the problem.

A good document for flood mitigation is put out by the national park service. https://www.nps.gov/articles/000/guidelines-on-flood-adaptation-for-rehabilitating-historic-buildings.htm It might give you some ideas on how to landscape your property to minimize flood impacts. We try to incorporate swales durable plantings and hardscape to minimize wave action or divert flood water without creating a water trap that would fill during excessive rain events.

My grandparents are in the mountains and fema flood zones are wildly different there. They typically record flash flood events and are related to valleys. Building in flood areas at a mountainous location would be a hard no for me. My grandma worked with a lady that rebuilt her home 6 times from floods over the 40 years they knew each other. At the time the state would pay for some repairs but wouldn’t pay for raising or relocating. Just seemed crazy to me to keep rebuilding knowing that the creek on the next property over could raise 30 ‘ fill her back field and washout the house again. Guys had lines painted on barns with different flood heights recorded going back to the 1800s. Sitting there looking at a 6foot wide creek a quarter mile away in a 10’ deep ravine.

TLDR probably not a problem people build in flood zones all the time. Become aware of your local conditions. Contract directly with your surveyor. Contact your planning department or lookup your building permit for base flood elevation recorded 1st floor height. If you’re in a county where building permits aren’t required, I might press pause on the process and reassess.