Whenever you tackle a renovation project, it always seems to cost twice as much as you thought and take twice as long to complete.

Construction surprises are one reason why construction budgets get blown. When the unexpected happens, it usually costs a lot to fix it.

One neighbor of mine recently began work on an extensive renovation project. She and her husband planned to gut the kitchen, rework the first floor, and add on a screened-in porch.

On the third day of the job, the crew demolished the concrete patio and began digging down to put in the footings for the porch. They heard a ping, and stopped.

Turned out, they struck oil – or rather, a century-old oil tank that was triple the size of the 800-gallon tanks that were standard in the area. The tank is completely filled (partly with oil, mostly with water), and will cost around $3,000 to empty and remove it from the property. If the tank had leaked over the years, contaminating their soil, the cost to remedy the problem could have been far higher.

What other sorts of problems might surprise you during a renovation? You could find that your home doesn’t have insulation in its outer walls. Spending an extra $1,000 to $3,000 now to blow in insulation will save you several times that much in energy bills over the next ten years.

Or, you might find that your plaster walls won’t withstand the rigors of construction. You may need to pay for extra demolition to get rid of the walls, and then pay to have brand new plasterboard installed.

If you tear out a wall, or even tear off some wallpaper, you might discover mold that has to be removed before work can proceed. Homes built before the mid-1970s might have lead paint. Or, you could have asbestos tiles or wrapping that has to be removed.

In almost every renovation project, some changes will have to be made to the plans while construction is ongoing. Homeowners who can’t make up their minds, or who change their minds about how something looks after it has been built can expect their construction budget to expand.

Building from the ground up typically means you’ll have fewer construction surprises than someone who is renovating an older home. But even when you’re building a brand new house, expensive surprises can happen.

For example, one Atlanta homeowner found that her property had been used as a dumping ground. When digging down for the foundation, the builder didn’t find dirt – he found garbage. The homeowner had to pay to have all of the garbage removed and the hole filled in with new material.

How can you protect yourself from construction surprises?

Unfortunately, there’s almost nothing you can do to completely protect yourself from construction surprises – so, you have to prepare instead.

When you’re planning your construction budget, make sure to set aside an extra 15 to 20 percent to pay for whatever surprises pop up. For example, if you’re planning on spending $50,000, be sure to set aside an extra $7,500 to $10,000 to cover anything unexpected.

Next, carefully review the plans ahead of time to catch possible mistakes. When my husband and I renovated our own home four years ago, the builder read the plans incorrectly. He ended up ordering mismatched windows for the laundry room. Instead of waiting for the right size window to be delivered, we ended up using just one window in the room, and building in a hanging unit for clothes. (It was a good solution, but we ended up with an extra window that we had paid for that we couldn’t use.)

Whenever you have to make a change to the plans, be sure to put your change order in writing, so the contractor knows exactly what you want. Be sure to sign and date your change orders as well, and either hand them to the builder directly or fax them to his office.

Finally, stick to your decisions. Changing your mind halfway through will give you sticker shock when the project is finished.

Next week, the oil tank is going to be removed from my neighbor’s yard. Fortunately, her contractor had other things for his people to do in her home for two weeks while this problem was being resolved. But for some homeowners, those two weeks would have simply meant their construction job would last two weeks longer.