S.T.
This is going to be a good, but tough, situation to work around - we did something like this 13 years ago. We look back at it now and can say it was probably the best thing we could do - and it still is - but it will add some bumps to you road. I'll spill my thoughts here and take from it what you will.
After my mom began having lots of health problems and she could no longer manage the family house we sold our little house and bought hers. We couldn't build an apartment for her until the house was sold so we lived in the same house, shared the kitchen and bath for about 6 months - until her apartment on th e side of our house, was ready. She still lives in her own apartment, on the other side of our living room, with her own kitchen, laundry, bath, entrance, etc. But she's right through the door if we need eachother. It's worked out really well when my kids were yo jng and had to stay home sick from school and when my mom came home from hospitalizations, surgeries.
Even now that my kids are now teens they'll still go to my elderly mom for comfort, they help her (garbage, get things from storage, help make her bed, etc.) and she gives them extra love and support. It's nice for them to get a good word from someone who loves them when they think their parents don't (after discipline, etc.)
It was a VERY rough transition. It's very hard for a parent to be neutral in disagreements between husband & wife. My mom could not keep her 2 cents to herself - which was a HUGE problem for my husband, and in turn for me. Of course, she ALWAYS took my side. I had to have my aunt talk to my mom about it - as she was someone my mom respects. It took a while but my mom has learned to butt out.
My mom also had to learn to respect our private time and our family practices. My mom had lived alone for about 15 years by the time we all moved in together - and she had forgotten all about compromise and give & take. It was very tough for her to suddenly have to get used to how other people do things - everything from mealtime, to how often towels / washcloths should be launderd. Since I worked full time at that point she had a very tough time that our cleaning standards were not the same as hers. I had to remind her that although I've love an immaculate home, I was not going to stay up until midnight to clean the house.
For my mom it was a loss of freedom - she felt that she no longer had control of things that she had controlled since she got married 45 years earlier. Suddenly her daughter was in control of stuff.
My husband reverted to a rebellious teenager in response to having my mom living in the home - and he suddenly began going out to play cards with the guys every saturday night until 2 - 3 AM (that went over big with my mom.) ;o) He now admits he handled it really, really badly - but he was reacting to feeling like the dynamics suddently changed to 2 against 1.
It took a lot of calm discussions while out to dinner (my mom babysat - so that was a positive), and we were both able to discuss what was really going on deep down. It was a lot for my husband to deal with and I did not give him the credit he deserved for dealing with a tough situation.
In hindsight I would have had my aunt spend more time "counseling" my mom as to how to deal with this new family dynamic - and I would have spent more time making sure my husband realized I was on his side not my mom's.
Now my mom is 80 and battling recurrent ovarian cancer and we realize that she'll not be with us much longer. My husband lost his father suddenly last year and his mom is now in a nursing home. We've gained perspective about the fraility of life, and about our loved ones, and we look back now and know it was a good thing that we made this family living arrangement. We've been able to be there for my mom, she has been there for our kids and as a support for us when our lives got turned upside down due to a serious car accident, when my husband became a NYPD office and began working weird hours, after 9/11 and my husband was working long hours, during a job loss a number of years ago, etc.
I could write another 20 pages about how this has been an overall good experience through various time in all of our lives. I also think it's a great example for our kids to see how to honor our parents and the example will benefit us in our old age. Bottom line is that your marriage is most important and you have to jealously protect it.
If you need any words of encouragement or advice feel free to reach out to me through this venue.
Best of luck!