What should you find out about a lot before you buy it to build on?

Published September 28, 2026

Summary

Find out whether the county will let you build a house there at all, whether it can be served by sewer or will need a septic system, and whether it sits in a high-risk flood zone. Those three answers decide whether the lot works and how much has to be spent on it before a foundation goes in. If there's no public water, add a fourth: you'll be responsible for your own well.

One county's list, before a permit

Rules on land use and access are set locally, so it helps to see a real list. Deschutes County, Oregon, tells people planning a one- or two-family home to finish these steps before applying for the building permit, each where it applies:

meet land use requirements through the planning division
have an approved septic system, or approval to install one
have driveway access, which may mean applying for a driveway permit

The county also asks owners to confirm that the property is a legal lot of record, and a septic installation permit there requires a valid site evaluation that names the type of system approved. A seller's listing won't settle any of this. The county will.

No sewer line means a septic approval

More than one in five U.S. households rely on septic or other onsite systems, and 9% of homes built for sale in 2025 had their own septic system.

EPA doesn't regulate single-family septic systems. In most states the local health department issues the permits, and under most programs the permitting agency assesses the site to decide whether the soil can treat the wastewater. It also sets distances from buildings, driveways, property lines and surface water. Lot size, slope and soil conditions all affect which kind of system you can use.

The drainfield, a shallow covered excavation where the treated water soaks into the soil, takes up part of the lot. EPA says not to build structures on it without checking with a local designer or the permitting authority, and to keep septic tanks away from areas that flood or pond. Where the house can go often follows from where the drainfield has to go.

Flood zone, and what your lender will require

FEMA's Flood Map Service Center is the official public source for flood maps. Zones starting with A or V are the highest-risk areas; B and X mark moderate risk, roughly between the 100-year and 500-year flood limits.

In communities that take part in the National Flood Insurance Program, a home in a high-risk area with a government-backed mortgage has to carry flood insurance, and some banks ask for it outside those areas as well. Low risk isn't no risk. One in three flood insurance claims comes from low- and moderate-risk zones. The maps are updated over time, so the version that counts is the one in force when you buy.

A well is yours to look after

More than 23 million U.S. households get their drinking water from private wells. Those wells aren't covered by the federal Safe Drinking Water Act or by most state laws, and the owner is responsible for the water being safe. In a USGS study of about 2,100 wells cited by EPA, roughly one in five had at least one contaminant above a human-health benchmark.