Whether you're checking the property over during a tour or are walking through with a home inspection professional, there are a few serious red flags to take notice of. Ignoring these costly warnings mean your dream house could wind up being an unexpected nightmare.

1. Cracked Foundation - Every tiny crack and crinkle you see isn't cause for panic and is usually the result of normal settling of the home over time. However, horizontal cracks or wide cracks of a quarter of an inch or larger are a sign of potentially serious foundation damage. How costly is this red flag? Depending on the cause of the damage, a fix could average between $5,000-$10,000. If excavation is required for the repair, the price can exceed $10,000 quickly.

2. Roof Damage - From the outside, curling shingles and missing shingles are a red flag. From the inside of the home, water stains on the ceiling or fresh paint attempting to hide water damage are red flags. How costly is this red flag? If the existing roof has only one layer of shingles, damage can be repaired and a second layer of shingles added. If the existing roof already has two layers of shingles, both layers have to be completely removed and a new roof installed from scratch. Depending whether the roof has one layer of shingles or two determines the cost, which averages from $2,000 for a roof with one layer of shingles to $10,000 for a roof with two layers of shingles already present.

3. Outdated Electrical Wiring - If the home you're considering was built prior to 1970, it's important to determine if the electrical wiring has been updated since the home was constructed. Homes built prior to 1970 are more likely to have aluminum branch circuit wiring and/or knob and tube wiring. Both types of wiring are high risk for causing house fires. How costly is this red flag? Upgrading the electrical panel alone averages between $1,000 to $3,000. Rewiring the entire home can cost $10,000 or more.

4. Old Plumbing/Pipes - Older galvanized pipes or visible corrosion on exposed piping are cause for concern. Homes constructed between 1978-1995 frequently used pipes made from polybutylene. In the 2000's, it was discovered that this pipe material degrades over time, resulting in major leaks and damage. If the home you're falling in love with has polybutylene pipes, they'll need replaced immediately. How costly is this red flag? To re-plumb a home with modern PEX piping averages about $5,000. To re-plumb with copper piping averages about $10,000 or more.

5. Sewer Problems - Red flags of sewer problems include heaved walkways, evidence of flooding in basements and standing water in the yard not caused by recent rainfall. Any one of these red flags could indicate a sewer blockage or a sewer line break between the home and the sewer main in front of the property. How costly is this red flag? Sewer line repairs can cost as much as $100 per foot. If the break or blockage is closer to the sewer main than the home, the cost can quickly escalate to several thousand dollars.

There are lots of things about a home you're considering that are inexpensive to update or upgrade, such as flooring or paint. However, these 5 issues are serious and costly red flags you should never overlook.