Real-estate agent Nancy Raddohl has seen the ugly side of home improvements done wrong. Worse still for home sellers hoping to impress Raddohl’s buyers, her customers have witnessed the alleged improvements, too.

“A buyer’s first impression is ‘I don’t want to go any further’ into the house. They won’t get beyond the front entrance,” the suburban Minneapolis-based Raddohl of ReMax Carriage House said.

Raddohl’s candor is a zinger through the heart of homeowners who may think their handiwork is up to professional snuff. Quality upgrades can add value to a home, but shoppers are easily turned off by badly executed projects. The listed price can drop accordingly.

Errors are especially onerous in highly visible glamor rooms such as kitchens and baths. Even minor miscues such as sloppy painting can have a negative effect on buyers.

So before you power up your hand tools, consider this: Sellers risk thousands of dollars of their asking price because of shoddy workmanship. It’s common lore among real estate agents that buyers set the value of a home, not the seller.

I call it “the home-improvement penalty.” Thinking your end result is good enough won’t cut it in remodeling circles.

Of course, this deflates do-it-yourselfers who puff their chests in pride at a job well done. So what if drywall seams show or spacing between bathroom tiles is a little off? To buyers, however, any glitch is a big deal. Even little errors may cause buyers to turn tail to the next property. Their perception is the seller’s reality: If one thing is wrong, are other problems far behind?

Homeowner mistakes can be pounced on by real estate agents and contractors. Raddohl won’t hesitate to make poor work a negotiation point to shave thousands off the listed price. Using the example of an amateurish countertop, she said she would advise buyers to press the issue.

“I’d ask sellers to have work redone, and it could cost $5,000 or more,” she said. The repair’s costs come straight out of the seller’s pocket.

Contractors are equally keen to benefit from improvements gone bad. If a bathroom project goes down the drain, a contractor knowing the homeowner is over a barrel for needed repairs may be less inclined to negotiate and can charge top dollar.

On top of materials and labor, homeowners may also be stuck for structural repairs if a job that really went haywire caused damage to portions of the home.

If a project doesn’t fit a contractor’s schedule, the eyesore could serve for weeks as a reminder to not take home-improvement projects lightly.

There are other risks, too. Homeowners may inadvertently skirt local permit requirements. Even if the work is passable, appraisers or building inspectors may nix a project weeks, months or years after completion and require work be redone to meet local codes. Again, homeowners will dust off their checkbook to pay for mistakes.

The bottom line: Homeowners must bluntly assess their DIY abilities. Impressive work involves more than skills; it also requires the right tools, materials, planning and project management.