We've been on every side of this now.
1. When I was 22 and DD was 10 months, I moved in with my parents to get back on my feet. I stayed for almost 2 years, but even after I moved out we stayed very close.
2. When DH's father died, his mother came out and lived in a nursing home nearby. When the nursing home had a decline in their capacity to handle her (dementia), we bought a house with MIL's money (DH has no sibs) large enough for her, caregivers, and our family. It's actually two spaces on one property, one ground floor "apartment" of sorts with 2 bedrooms and an efficiency kitchen/dining room, and a bathroom with shower, that was very easy to make handicapped accessible, and one large house. My sister and her husband moved in to be her caregivers at night and part of each day, we had a service in for a few hours per day, and my husband and I did part of her care each day.
3. MIL died in 2009, peacefully, in our living room, and shortly after that my sister had a baby. So now, our constellation is this:
Mom and Dad live half a mile away
Sis and BIL and my niece live on our property and pay rent (or clean in lieu of rent) for the apartment.
I live with my husband and two daughters in the main house.
4. My adult daughter (just turned 18, out of high school, not in college yet) is living with us rent free at the moment, but she's so helpful that it's not an issue.
What it boils down to is this:
Everyone, and I mean EVERY adult in the living situation needs to be clear on what their boundaries are and what other people's boundaries are, and what needs to happen to keep everyone content with the living situation.
When I was living with my parents, I had a bedroom that was for me and my daughter, and used the common spaces with them. They had a lot of boundaries about what I could leave where, etc. etc. in their space. I needed the freedom to not have them in my bedroom. And we worked out a lot of personality issues. This living arrangement would not have worked if i had not been paying some rent to them, because it put me on an "Adult living with adults" footing rather than "We're paying for everything, do what I say" adult/child relationship.
Babysitting boundaries: My parents said that in order to feel good about saying "yes" to babysitting, they ABSOLUTELY needed to be able to say no without guilt. This is actually great for me, because I know if they say yes, they mean it.
Parenting boundaries: My mother is absolutely within her rights to insist that diapers be changed either in the bedroom someone lives in, or in the bathroom at her house. But I'm absolutely within my rights as a parent to not have people feed my kid things I don't want her fed, or to discipline my child in ways I do not consider acceptable. And just because I was living with them or visiting them didn't mean they had the say-so over how I parented my young children. We sorted that out pretty quickly.
Cleaning boundaries: We've run into this in every constellation. I'm not a particularly tidy person by nature, but how I am in my own space is different from how I need to be in other people's spaces. Likewise, because I'm not a tidy person, and I'm essentially paying people to clean my space most of the time, I cannot stand it when my sis and her family leave their stuff in the main house, and we set boundaries about that and it's a constant thing to keep enforcing them.
Food and money boundaries: Much the same thing. Who's paying for food? How is it being purchased? Are meals being shared? If someone buys something for their own use, how do they "stake a claim?" While my sis was being a caregiver to MIL, we were paying for their food (room and board were part of their compensation)... but that didn't mean that they could just have everything from our fridge. I was perfectly willing to buy them good healthy food, and we often, almost always for a long time, ate meals together, but if I buy smoked salmon at $15 per pound for special treats for me and my husband, I don't feel obligated to make that open season for everyone. So while we were buying their food, we kept a separate fridge in my husband's office for "off limits" foods. Likewise, if food is in their fridge, we don't go in there, ever.
Utilities and money boundaries: We include utilities with rent, but during the winter I find it stresses me out because they often keep their place wastefully hot. It's a good idea to look closely at how the utility bills change when you move in, and find thermostat settings that work. Who's paying the utilities? How do people like to keep their environment? What about Internet? Phone? I'm on the verge of having them get their own phone line, it's not quite annoying enough most of the time for me to do that, but just about...
Would you be contributing to the mortgage payment? If you pay rent, are you paying rent, or helping with the mortgage? Is there any expectation that if you contribute to the mortgage you would have any ownership interest in the house? In general, the answer is "no", but I know people have said that they helped their parents pay mortgage and were extremely upset when the house was sold and they were not reimbursed a share in balance with their contribution. IMO, if you're paying rent to your mother, you're not "buying a share in the house" unless you have a written agreement that says so. And if you're not paying rent, what does that mean for the relationship, if anything?
When it comes down to it, if you do it, you'll need to sit down not only before you move in, but after you move in, and on a regular basis after that, and say, "okay, how is this going to work? How is it working? Is there anything we could do to make it work better? Is there anything that is actively NOT working?" If you don't have the kind of relationship where you can have that sort of talk with your mother, it's a risky, risky move. With my parents, we can sit down and talk about things that aren't working and know that we're not looking to criticize, but to find ways of making it work better. Back when every talk of "this isn't working" was derailed into a "Oh, I'm a bad person" conversation, it was a much, much harder thing. When we got past that, and started looking at discussions of things that weren't working as opportunities to make them work better, the whole family got a lot happier.
I know that my relationship with my teenager is still very much parent-child, though she's an adult, because there was no real transition from "school-going kid" to "living at home kid". On the other hand, she's such a fundamentally helpful and generous person, and is doing SO much for us right now during my current pregnancy, that I make a huge effort day to day to keep it in perspective. She helps us, we'll be helping her when she's at college, it's not quid pro quo, but it's in balance.
And that balance is critical to making any sort of long-term extended family living situation work.