The first thing any savvy do-it-yourselfer should take into account when considering a new project is that it will probably take twice as much time and three times as much money as you thought.

Or maybe it's three times as long and twice as much money.

Either way, there's a good reason why it's true: DIYers make mistakes.

Lots of them.

But you can learn a lot from mistakes. For example, whatever it was that my husband did to make all the outlets in the kitchen blow at the same time -- don't do that again. The bad news is that mistakes always wind up making your home-improvement project more expensive and more time-consuming than you want it to be.

With that as a given, we asked home-improvement experts around the country for the most common mistakes they see.

Top 10 mistakes by DIYers

Here's their consensus • Not taking out the required permits. • Starting a job without the necessary tools and supplies. • Inadequate preparation of the job site. • Skimping on materials. • Using the wrong paint. • Improper preparation of walls for painting. • Unsafe job conditions. • Inaccuracy. • Working beyond your limits. • Failure to get a clue.

1. Not taking out the required permits. Considered a bother at best by many DIYers, permits actually serve a greater purpose than just raising money for the government. "People in permitting offices aren't evil," says Lou Manfredini, the official Ace Hardware "Helpful Hardware Man."

"They're there to make sure the job is done right and you don't hurt yourself." Plus, for some jobs, such as putting in a wood stove, you need proof of the permit or your insurance carrier won't cover it. Not sure if your job requires a permit? The rule of thumb is that you need one for anything larger than painting and wallpapering. It doesn't hurt to call the building department and ask.

2. Starting a job without the necessary tools and supplies. Nothing slows down a job more than not having all the materials you need. Manfredini says the reason the pros can do what they do is that they buy quality tools. "There's always a bargain bin," he says. "It's not a wise investment. You lose time and money."

3. Inadequate preparation of the job site. If you do a small addition, suppliers will be delivering materials. You don't want them out of order or exposed to the weather while you are working, says Ed Del Grande, host of the DIY Network's "Warehouse Warrior" show.  Beware: They could be stolen if they're not properly stored. (If you have a septic tank, make sure you know where it is. If a supplier delivering materials in a heavy truck drives over it, you could be looking at a cracked tank. Yuck.)

4. Skimping on materials. Barbara Kavovit, owner of Barbara K Enterprises in New York, says she often sees DIYers use 1/4-inch drywall for building walls instead of the minimum 5/8-inch or 3/4-inch if you want a good sound barrier. The same rule applies to plywood for subfloors. Go with 3/4-inch. It creates a much stronger floor, especially if you're installing wood floors over them.