Remodeling your home is a big investment that takes a lot of planning. You need to design the project, price out materials, get bids on labor, and figure out what budget you are comfortable with before you even begin. In an ideal world, your project runs smoothly and finishes on time and under budget. In reality, that is rare. There are many costs you may not factor into your original plan. These are the top five potential costs hiding in your walls.

1) Water damage

There are many potential sources of water damage, from roof leaks to plumbing issues and condensation within an HVAC system. Often you don't realize you have a problem until the damage is already done. Water damage often leads to wood rot and mold, which means extra demolition, labor and material to replace damaged wood, and in the case of mold, a costly remediation. A slow leak that has gone undetected for a long time is a perfect environment for mold to grow on both wood and drywall. Because mold can be so harmful to your health, the area needs to be sealed off, the materials disposed of properly and the air scrubbed. This process can cost thousands of dollars.

2) Insect destruction

Several types of insects cause damage, like carpenter ants and honeybees, but termites are by far the biggest offender. They are responsible for billions of dollars in property damage every year. They eat right through wood, and the channels they create can weaken the structural integrity of your home. Near windows and doors, look for wings and droppings as signs of an infestation, although often you won't see any clues at all. Consistent termite treatment may have killed active infestations, but that doesn't tell you what damage they caused in the past. A good inspector will alert you to an issue when you buy the home, but it is not something a contractor looks for when writing an estimate.

3) Contaminants

The largest contributors in this category are lead, radon and asbestos.

Lead. If your home was built before 1978, lead paint is a potential concern. Lead abatement permanently eliminates lead-based paint hazards and can involve specialized techniques not typical of most residential contractors. Exposure to high levels of lead can cause anemia, weakness, kidney and brain damage, and reproductive issues. If you suspect a home you are buying may have lead paint, pay for the testing during the inspection period. Waiting until a remodel down the road can be costly to both your health and your budget.

Radon. Radon is a radioactive gas that cannot be seen or smelled, and it is the number one cause of lung cancer among non-smokers. It can be found in the soil and rock beneath a home, in well water and even in some building materials, and it enters through crawlspaces and cracks in the foundation. Homeowners can run a passive test, placing sample media in a specific location for a set time and sending it to a lab, or use an active monitor for an instant reading. If the air reading is high, test the water too. Remediation involves sealing cracks, holes and entry points in the foundation, and treating water with a point-of-entry device.

Asbestos. Asbestos is a group of six naturally occurring minerals made up of heat-resistant fibers. It is most likely to be found in wall and attic insulation, old vinyl flooring and adhesives, roofing and siding shingles, and textured paint and patching compounds such as old popcorn ceilings. Testing suspected materials can cost $500 to $1,500, and removal in a limited area averages around $2,000. A whole-house remediation can run into tens of thousands of dollars. The work must be done by a specialized, environmentally certified crew, with the area sealed and negative air fans running so particles are not released. Asbestos can cause mesothelioma and certain cancers. If contaminants are found during your remodel, this is one area where you cannot cut costs.

4) Old work, new laws

The older your home is, the more likely you are to find something that doesn't look quite right. Work that requires a permit today didn't always require one years ago. A previous owner may have done it themselves or "known a guy." If that work doesn't conform to current code, it may have to be redone, even if it isn't directly part of your remodel. Think questionable wiring inside walls, outdoor rooms converted to indoor space without proper structure, insulation or HVAC, or laundry rooms without proper dryer ventilation. If something doesn't look like part of the original floor plan, have an inspector or licensed contractor take a closer look.

Depending on the work you are doing, specific items will need to be brought up to code. If you are remodeling a kitchen and doing electrical work, the city may require you to update, and possibly add, smoke detectors throughout the home. If polybutylene pipes are discovered during a bathroom remodel, you may need to replace them with copper or PVC, since polybutylene is prone to failures and leaks. Adding an addition or pool may require extra foundation support. A typical contract assumes pilings up to about 20 feet deep, but near the water or in a state like Florida with a high water table, you may need to go much deeper, which can add tens of thousands of dollars.

Most expensive of all, many municipalities require the whole home to be brought up to code if the renovation exceeds a certain square footage or a certain percentage of the home's value, often 50%. This gets especially costly in areas prone to hurricanes and flooding, where codes have changed significantly. For example, a waterfront Florida home built 50 or 60 years ago and remodeled today for more than 50% of its value could need new wiring, new plumbing, new windows, a new roof and ties, and to be raised several feet from its current elevation. At that point, demolishing and building new may make more sense. Make sure your contractor knows the current codes and limitations in your area. Your scope of work may not even be feasible, or you could end up far over budget.

5) Structural changes

Many contractors can determine which walls are structural, but without original plans it can be an educated guess. Removing non-load-bearing walls to open up a space is fairly simple and relatively low cost for the value it adds. A load-bearing wall supports the weight of the roof, and removing it can get expensive. A small opening may be supported with a wood beam, but a large opening will likely need steel. That greatly increases material and labor costs, and because steel is heavy, you may also need additional foundation support. This requires an architect and engineer, and additional piles can cost thousands of dollars.

Even when a wall is non-structural, you can find things inside it, like a vent stack, waste line, air conditioning drain lines or a gas line, that are costly to reroute. If you have your home's original plans, share them with your contractor so these costs can be accounted for up front. Be wary of a contractor who simply looks around, listens to your wish list and gives you a price. A low number may be tempting, but that is how projects end up with change orders and escalating costs. There is real value in a contractor who takes the time to evaluate the project thoroughly and has trade partners put a second set of eyes on the scope.

Plan for the unexpected

These issues can have a big impact on the overall cost and timeline of your renovation. The right contractor can help you minimize surprises by spotting potential issues before demolition and explaining what to expect. It is always wise to keep a 10% contingency in your budget for the unexpected. It is frustrating to spend money on things you will never see, but your health and safety are worth it, and these items should always be taken seriously.