A shortage of real estate appraisers is delaying home purchases and sales in parts of the US.

The number of appraisers—the experts who evaluate properties to determine their current value—has plunged by 20% since 2007, with further annual declines of 3% expected per year for the next decade, according to the Appraisal Institute, a trade ­association.

That means real estate deals could take several weeks longer to close, ­especially in rural areas. It also could lead to less accurate appraisals as appraisers need to cover multiple states and localities that they may not be ­familiar with.

To avoid this, ask about an appraiser’s experience in your area and never agree to an automated appraisal in which a software program generates a property’s value based on comparable properties without an expert actually looking at it.

For buyers: Ask your mortgage lender whether there’s an appraisal delay in your area…whether a 30-, 45- or 60-day interest rate lock is adequate…and how much it costs to get an extension.

For sellers: Talk to your real estate agent to figure out how a delay might affect the purchase of a new home if it’s contingent upon selling your old one first.