I've fought, and won a property tax dispute two years ago with the city and here is what I learned.
1. R Audi is right in that there is a percentage error, approx 5 or 10% of the value of the house so 400 000 = 20 000-40 000 dollar range of error, if you are within this range you are SOL
2. The city bases its valuations on SOLD prices as close to July 1st as possible, and then physical condition (did you devlop a basement? add a fence or deck?) until December 31st of the previous year. They also compare your house with other houses on your block, community, quandrant, and finally against everyone in the city to determine property value. Obviously houses in your own neighbourhood carry more weight then those accross the city. The last part of the evaluation is the features of your home, such as a fireplace, attached/detached garage, lot size, and house size. This is where the city "gets" you. They will not tell you how much they value a fireplace or garage or how much every sq/meter adds or subtracts to your value, you have to do all the calculations on your own.
3. If you decide to dispute the assessment you will be contactacted by your local asseser and will have 5 minutes to explain why you disagree with the assessment. They will *almost always* tell you why you are wrong and explain how they arrived at their number. Very very few assessments are changed at this stage unless there was something obviously wrong like a computer error. You then can decide to fight the assessment (30 dollar filing fee, returned IF you win) or forget the whole thing.
4. Fighting the assessment means getting as much information as possible, lots available on the city website but you'll also need access to specific home sale history information for your area which means you'll need a relator to get it for you. There may be a cost involved in this, depending on who you know. The city will send you their brief of how they valued your property and why they think they are right and you are expected to do the same some weeks before your actual hearing.
5. At the hearing you'll be surrounded by mostly old to middle aged men that are totally unprepared and are just there to complain. If you follow this route you will lose. The most important part is to be prepared with actual evidence in the form of sale history that the city has over valued your property. You will argue your points and the city will argue theirs and then the board will make a decision. If you win you get your 30 bucks back plus lower taxes and if you lose your out a whole lot of time and 30 bucks.
6. In the end after many hours researching and putting together a brief and then wasting a morning at the hearing I saved myself about 50 - 100 bucks in property taxes over the course of that year. Not worth it in my opionion, and even though they have gone back to over valuing my house each year since I really cant be bothered to fight them for that small amount of money.
So look at your property and and see how much of a difference fighting and winning would make. In 99% of cases it won't be worth it as in my opinion the time needed to research and prepare to WIN makes the whole thing pointless.
You start with a bag full of luck and an empty bag of experience. The trick is to fill the bag of experience before you empty the bag of luck.