You'll need to check with your local planning and zoning office for the answer. Some areas view it as a regular subdivision if it's not a family subdivision (family dividing & living on the property). You may have to submit an engineered subdivision plan to accomplish it and that has all sorts of headaches and expense!
We did one just like you are planning and the city wanted curb & gutter put in. This was on a DIRT road & unfortunately was under the cities planning jurisdiction. The county maintained the dirt road and the city provided police protection.
Sellers may do this for you if the cost to do so is paid by you. There's really no benefit for them as it's your desire to subdivide.
Best advice is ask your local authority & get it in writing.
Good luck!
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"There are people who make things happen, there are people who watch things happen, and there are people who wonder what happened. To be successful, you need to be a person who makes things happen.." - James Lovell- Astronaut