On an unsewered lot, the septic system is what determines whether a house can be built, how large it can be, and where it can sit.
What to establish before committing
Does the lot perc, and does the soil evaluation pass, to the county's standard, with the health department's review.
Where the field and the required reserve area would go. Those two areas constrain the house, the driveway, the well and any outbuilding.
How many bedrooms the site can support, since that is what sizes the system and therefore what house can be built.
What system type the site requires. Conventional, mound, sand filter, aerobic or drip — a large range in cost.
Where the well would go, with the required separation distances from the field, the reserve, and any neighbouring system.
The contingency
Make the purchase contingent on obtaining a satisfactory soil evaluation and health department approval for a system serving a stated number of bedrooms.
That is the standard protection, it is routine in rural transactions, and it is what turns "the agent said it percs" into a fact.
If the seller resists a testing contingency on an unimproved lot, that is itself information.
Testing properly
In the area a field would actually occupy, not a convenient corner.
Multiple locations, since soils vary over short distances.
In the season the rules require, which in many jurisdictions means the wet season when groundwater is highest.
By whoever the jurisdiction requires — a licensed soil scientist, an engineer, or a certified evaluator — with the health department notified or witnessing.
A test done informally, out of season, or by an unqualified party may not be accepted, and the result may not hold.
What the county already knows
The health department holds records for approved systems in the area, and often prior evaluations on the lot itself if anyone has tried before.
A lot that has failed a previous test is a lot with a file, and asking is free.
Soil survey maps from the NRCS give a broad indication of the soils and their limitations for on-site wastewater. They are not a substitute for a site test and they are a useful early screen — a lot mapped as severely limited is a lot to test before falling in love with.
The costs to budget
Testing and design — a modest sum on a conventional site, more where monitoring wells or an engineered design are needed.
The system itself, which on a difficult site can be a substantial fraction of the cost of the land.
A well, if there is no water supply, with its own testing and its own uncertainty.
Together these can change the economics of a lot considerably, and they are the reason two apparently similar lots sell for very different prices.
The lots to be careful about
Small lots, where the field, the reserve, the well and the house must all fit with their separation distances.
Steep ground, which limits where a field can go.
Low-lying ground near water, where the seasonal high water table is the constraint.
Lots with a failed test on file.
Anything where the seller offers a "perc letter" from years ago, since rules change and the letter may not reflect current requirements.
The one sentence worth remembering
An unimproved lot without a current, approved soil evaluation is a lot whose buildability is unknown, and the test costs a very small fraction of the mistake it prevents.
Shared and community systems
Some rural subdivisions are served by a shared system or a small community treatment plant rather than individual ones.
That changes the questions entirely: who owns it, who maintains it, what the assessment is, and what happens when it needs replacing. Those answers are in the association documents, and they are worth reading before rather than after.