o you thought buying a house was expensive? Think about this: It's going to cost you four to 10 times as much as what you paid for your home to ensure yourself of a comfortable retirement.

The amount is so staggering that few of us bother to wonder if our nest egg is really big enough.

It's time to wonder. Traditional "retirement needs analysis" calculates your current expenses and then assumes you'll need somewhere between 70 percent to 100 percent at retirement.

Such calculations are great as a wake-up call, but they ignore the most important step of all in retirement planning - imagining what you want retirement to be like. Plainly put, the amount of money you'll need in future years depends on the lifestyle you plan to lead. Will you travel around the world? Is there a hobby that you've never had enough time to develop? Will you continue working part-time?

The reality is that pre-retirement expenses often have little or nothing to do with a post-retirement budget because we tend to think of retirement as a "new start" in life, not "winding down." Going back to school, traveling and other new expenses may be in your retirement agenda. For instance, people planning for retirement 20 years ago would never have thought to budget for a new computer every few years, along with monthly online service fees. Even seemingly small lifestyle decisions, such as paying $1,000 a year in tennis club fees, can make or break your retirement piggy bank.

The way to make retirement planning realistic is to forget the numbers at first and use your imagination to construct a detailed picture of the way you want to live.

We tend to think of retirement as one never-ending sunset, stretching from age 65 or so out to the horizon. In reality, retirement has two distinct phases. The first phase consists of the years during which you and your spouse are healthy and living an independent life. You may even be working part-time to bring in some extra income. The second phase consists of the less active years, when your health is not as good and you need assistance in your daily life. Stage two can be much more expensive than stage one because of the cost of health care and increasing longevity. Instead of earning some extra income, you're paying someone else.

There are a variety of questions to consider as you try to imagine each phase. If you're married, you and your spouse should answer them together. The best way to gauge the financial impact of each stage is to complete the retirement expenses calculator twice - one time for each phase.

To help you envision your life in retirement, consider the following questions and the financial impact of each:

In Phase Two, your health may decline or you may just slow down, but the result is that you need help in daily living. When you consider this phase, it's a lot harder to think about your life and lifestyle. But it's critical that you do so now, when you can plan for it. Use the retirement expenses calculator to estimate how much this phase will cost.

With some assisted care facilities and nursing homes costing $2,000 a month or more in the late nineties, it's clear that for many of us, 80 percent of pre-retirement expenses just won't be enough in the later retirement years. Here are the questions to ask yourself:

Answering the questions above, difficult as they may be, serves two critical purposes in retirement planning. First, it helps you put realistic numbers together to help you figure out how much in retirement assets you'll need. Second, and even more importantly, you've visualized your retirement, possibly for the first time. This brings retirement from the murky future into the clear present, so that you're more motivated to make changes necessary today in order to live the way you want when the time comes.