Plan for the worst of every scenario.
This is what happened to my friend.
Her Mum put all her money from the sale of her small Northern home into my friends house, contributing to the overall refurb and building a beautiful studio flat as an extension. All went v well - the Mum made her own local friends and activities, kept herself to herself except when invited, but was also a helpful source of childcare for the two rising-teen Dds.
Then my friend died in sudden and tragic circumstances. So her widowed DH was living with his MIL. Good to begin with , they were close, got on, she was a huge support with the Dd.
5 years on, he is seeing someone, wants to move, can't buy her out at a level that enables her to buy her own place, she cannot move back North - too old now, all friends from there gone. It's a big unhappy mess.
What if something happens to one of you? What if you want to or need to move for work? What if you divorce? It's all very well saying these things won't happen, but what if they DO? What would the plan be?