But sometimes it's not that simple. There might be a reason the job was offered on a full-time schedule--the employer really does have enough work that it requires you to be there 40 hours or more. So if you say, "I want this job, but can you make it 20 hours?" for many jobs, it's more complicated for the employer than saying, "Well, of course, why didn't I think of that?"
So what do you do if the perfect job is in front of you, but the hours just won't work? How can you handle it if your prospective boss is unwilling or unable to budge?
When you're looking for a job, you always want to make things easier for your employer to hire you. If your employer says, "Then how am I going to cover these clients/hours/customers if you're not here half the time?", you make things much easier for him or her when you say, "Actually, I've got a plan for that."
This requires research, maybe even more than most people are willing to do when they're looking for a job. But I feel pretty confident in stating that if you show you've done your homework on your future employer, you'll stand out from the competition.
If you want to work fewer hours, you need to show your employer how the work will get done while you're not there. This is similar to what you might do if you were already working full-time and wanted to ask for a new schedule, but since you're not actually employed there yet, you don't know what responsibilities your new job entails. So you'll have to find out.
You can do this by perusing the job descriptions, interviewing current employees, asking the HR rep for more information, or requesting clarification at the interview.
Then you'll have to figure out a way the work can get done. Here are some possibilities:
1. a job share arrangement
2. hiring an intern
3. spreading the responsibility among other employees/departments
If you want to make this work, you're going to have to learn a whole lot about your prospective employer. But it's worth it for your dream job, isn't it?
Are they open to a job share? Do either they or you have a candidate in mind? What responsibilities do they usually give their interns, and are they allowed to take on more? Are there any structural changes going on within the organization that would make it easier to incorporate some new roles that traditionally have belonged to the job you want?
None of this guarantees you'll get your dream job, of course, but it can increase your chances. When you say you'll only consider part-time jobs, you might be putting limits on the type of work you'll get offered. But by including full-time jobs in your search and doing your homework to come up with a workable plan, you're increasing your options, and you may find that your dream job and your dream schedule are less elusive than you think.