Dear Carolyn: I am a 32-year-old woman who has been dating a wonderful man for a little over a year. We are now making lifelong plans together, starting with the process of looking for a new home to move into together.

My boyfriend has joint custody of his 5-year-old son, who until now, for logistical reasons, has lived solely with his mom. We are looking at homes with the idea that his son would start living with us half of the time. His son and I have a very good and ever-growing relationship; the three of us have been car camping together multiple times and I see him at least once a week (usually more). He's a sweetheart and I welcome the idea of having him living with us, getting to know him better and helping my boyfriend raise him.

What are the sort of things I should be prepared for, and my boyfriend and I should discuss, before we move in together with his son? Any big issues that couples need to address when moving in together when one partner has a child? Neither of us has lived with a partner before, and neither of us has lived with kids before. I want to make sure we're preparing as best we can for this major life change all three of us will be making!

Carolyn says: You can make sure you're permanent. Before the house, before the half-time custody, before the book on stepfamily dynamics, there is your relationship with the father of this little boy. Calling him your "boyfriend" says you're still just dating, or are in the gray area between "dating" and "lifelong," and that says forming this stepfamily is premature. Period.

That doesn't mean your life with man and child is anything but promising. In fact, everything that needs to be there seems to be: A man is changing his life to make room for his child (belatedly, but, OK); the joint custody and regular visitation suggest he and the mom are cooperating; you're getting to know the boy gradually, and you like and care about both father and son; you are eager to provide a home and willing to work to get it right. Good, good, good, and good for you.

But setting up this "and-son-makes-three" household before you and your boyfriend have made a life commitment is just fundamentally not in the best interests of the child. You may have no intention of becoming the first in a series of Dad's Live-In Girlfriends, and he may not remotely intend you to become that, but your best insurance against that outcome is patience. Don't gather under one roof and then commit. Commitment, then roof. Please.

It's more money upfront, yes, but see it as a fraction of what it costs to undo mistakes.

In the meantime, prepare yourselves by enrolling in a reputable parenting program together, both to prompt important conversations and to establish a go-to resource for when you're caught unprepared by a new challenge, phase, diagnosis, or anything else from the what-did-we-get-ourselves-into menu (which is hardly just for stepfamilies). The boy's pediatrician is a good place to ask for a referral, since the office staff will be familiar with local offerings.

You already sound caring and conscientious; please just add "careful" to the list.

E-mail Carolyn Hax at tellme@wash post.com, or chat with her at 11 a.m. Friday at www.washingtonpost.com.