Kenneth Harney.

In an interview, Andersen told me she is considering bringing the issue to the Illinois attorney general because it affects all property owners in the state. She has also been approached about turning the matter into a class action, which could touch millions of owners across the country.

In the suit, Andersen said that she has been trying to sell her townhouse, which overlooks a golf course and is in a prime location, for $626,000 — roughly what she paid for it in 2009. Houses directly across the street but with greater square footage sell for $100,000 more, according to her court filing. But Zillow’s automated valuation system has apparently used sales of newly constructed houses from a different and less costly part of town as comparables in valuing her townhouse, she says. The most recent Zestimate is for $562,000. Andersen is seeking an injunction against Zillow and wants the company to either remove her Zestimate or amend it. For the time being, she is not seeking monetary damages, she told me.