In the course of planning for retirement, you may be inclined to make certain assumptions that dictate how much you should save for that milestone. But these three assumptions, in particular, could prove quite problematic in the long run.

1. You'll be able to retire when you want to

Many people plan to retire at a specific age or age range. For example, if you were born in 1960 or later, you may decide to leave the workforce at age 67 because that's considered full retirement age for Social Security purposes.

But one thing you may not realize is that not everyone gets to retire on the schedule they set. In fact, an estimated 48% of workers wind up retiring earlier than planned, according to the Employee Benefit Research Institute, and if you wind up being one of them, it could wreck your senior years.

Remember, you never know if a layoff at work or a health issue that springs up later in life will impact your ability to continue holding down a job. That's why it's crucial to save for retirement earlier on in your career rather than count on playing catch-up later on.

Older man sitting on couch pinching nose

Image source: Getty Images.

2. You'll be able to work in retirement

Some people plan to work part-time in retirement to generate income. It's a good idea in theory, but it may not work out in practice. First, you don't know what health or mobility issues you might encounter later in life. Secondly, you don't know what your local job market will look like and whether it'll be possible to get hired somewhere.

If you're really intent on working during retirement, your best bet may be to find a job you can do on your own terms and at your own pace. That could mean consulting in your former field from home or finding another income opportunity you can pursue remotely.

3. You'll get your full Social Security benefit

After many years of work, it's natural to look forward to Social Security. But the monthly benefit you might think you'll be getting could end up coming in much lower.

For one thing, you may be forced to claim your benefits early if you're let go at work or land in a situation where you can't hold down a job, and by going that route, you'll lower your monthly payout in the process. Furthermore, Social Security may need to implement serious benefit cuts if lawmakers don't come up with a fix for its impending financial shortfall.

A good bet with regard to Social Security is to plan for those benefits to make up no more than 25% to 40% of your retirement income and save enough to provide the rest. That way, you'll be better prepared if your benefits come in lower than expected.

The last thing you want to do in the course of your retirement planning is fall victim to poor assumptions that lead you astray. Be realistic about the above items so you can plan around them and avoid financial stress when you're older.