How do we respond to a deposit gift that comes with a promised room for Mum?

Mum said, "Of course it's a gift. I just want to know there's always a room for me." My partner looked at the floor plan. The second bedroom was the room we had planned to use for both our work desks.

Her offer is £20,000 toward our first home. Without it, we would probably keep renting for another year. She lives several hours away and wants to stay for a week or two at a time. I said there would always be somewhere for her to sleep. She then asked whether she could leave clothes in the wardrobe and book visits without checking dates because it would be her room too.

I can see how my answer sounded like agreement. My partner had heard it as offering ordinary hospitality, and we haven't accepted or transferred the money. We also need our purchasing advisers to explain any proposed financial arrangement before we commit to it.

The immediate problem is what we have promised each other. Would it be unkind to offer agreed visits and a convertible guest space instead of a permanently reserved bedroom? I don't want to accept a generous offer on terms Mum thinks are already settled while we silently mean something else.