Image: Home garage sale © UpperCut Images, SuperStock

Bad neighbors aren't just annoying. They can cost you real money when it's time to sell your home.

A nearby property's overgrown yard, peeling paint and clutter can easily knock 5% to 10% off the sale price of your home, said Joe Magdziarz, the president of the Appraisal Institute and a real-estate appraiser with 40 years of experience. A true disaster -- a junky home in deplorable condition and a yard packed with debris -- could cost you even more.

"In reality, there's so much supply right now that people are just going to pass," Magdziarz said. "It might make your home unmarketable."

Magdziarz has personal experience with how a bad neighbor can hurt a home seller's chances. A few years ago, he and his wife were house hunting and found a property they really liked -- until they looked next door.

"He was a hoarder. There was junk all over the yard," Magdziarz said. "The last thing I want to see when I get home is a junkyard."

Even when real-estate markets were in better shape, messy neighbors caused problems. Kamie Dowen put her Harrisburg, Pa., home on the market five years ago but had problems selling because of a nearby property.

Liz Weston

Liz Weston

Toys littered the lawn, even in winter. The porch sported "a pumpkin that was two years past due," Dowen said. A garage door, damaged after the owner ran into it with his car, was never fixed.

"After I moved out and staged my home, it still took me over a year to get rid of it," Dowen said. "I had to sell it at my cost."

So what can you do?

Frustration can lead to guerrilla tactics. Jeanine Brydges Watt of Windsor, Ontario, got so fed up with her neighbors' yard that she waited until they went on vacation, then mowed the lawn and threw out the trash, which included old diapers and split-open bags of garbage.

Watt said she wasn't worried about being arrested for trespassing. The messy neighbors were renters and probably thought their landlord had done it, she said. And Watt's other neighbors were thrilled.

"If they had been asked, none of the other neighbors would have ratted me out," she said. "They were happy we cleaned up the eyesore."

You may not be willing to risk arrest, but there are other tactics you can try if a neighbor's property is hurting your home's value.

If your neighbor is elderly or disabled and simply not able to maintain her property, for example, you may be able to help her find free or low-cost services that can help. Habitat for Humanity's A Brush With Kindness program offers exterior painting, landscaping, weatherstripping and minor repairs to low-income homeowners who can't care for their homes because of age, disability or family circumstances.