Hire a local architect to help and protect you.

I think you need first decide the type/size (e.g. single-story or multi-level) house you want to build, restrictions, CC&Rs, etc and understand if it's where you want to live. You need to know, for example, if that's a setback line or a line beyond which you can't build/disturb, or something else that could affect what you want to build. If that's the case, then that lot looks much less attractive especially if you want to build a large single story.

The plat maps should have a key defining what all all lines/symbols represent. Here's an excerpt from my lot map that my neighborhood developer provides. It's more detailed than what you may get from the county, but it illustrates information describing lot size, boundaries, easements, pad size, etc. You're showing only a portion of your map, but there should be a key showing what everything means.

You should also be able to go to your country's website and pull the maps with all the detail that you'd ever need.

Like @Mark Bischak, Architect said, talk to an architect. Also talk to a builder, read the neighborhood guidelines for the subdivision and any for the specific lot that you're considering (and then read them again).

The area beyond the dashed line appears to be part of the "Variable Conservation Mitigation & Drainage Easement." I've experienced a couple of properties with drainage easements and agree with the above comment that you need to learn about the setback and building restrictions. In my experience, these have always been quite significant--and sometimes costly--requirements.

After seeing @tendrac's comment and taking a second look, I have a strong suspicion that you won't be able to build past that line. Here's information from the EPA discussing Conservation Easements that may help you make an informed decision: https://www.epa.gov/system/files/documents/2021-11/bmp-conservation-easements.pdf

If the easement applies, then I'd walk away from it. For me, the restrictions, if present, would place the home too close to the street and not allow a decent backyard (Plus, I bet the builder is charging a premium.) With that said, if you love the location, perhaps you can ask for a discount given such a large and invasive easement.

As others have said, the plat map explicitly states what the dashed line is...

"Variable Conservation Mitigation & Drainage Easement"

@patrick A. I wish we had something similar as to what you have. Unfortunately we do not. Our county provides the bare necessities and leaves it to the expense of the property owner of future buyer to get the information.

Thanks to all who have taken the time to comment. I get a faster and more knowledgeable, honest response from this board than I seem to get from builders/developers.

I have just heard back from the developer and he states, 1003 will also have a conservancy easement across the back which can't be mowed or sprayed and must remain natural. To me that means weeds and snakes. Lol. We are walking away and in search of another lot.

If I were you I’d walk the lots with all the rain then can see whats mushy when wet !!!! And see how high up that creek goes !

I was about to say what she said ⬆️