Daughter living in 2nd property

[HumanRemains]

Hi! I'm hoping someone can give me a bit of advice.

I currently own my own 1 bed flat (no mortgage) and am due £100,000 inheritance from my late father within the next few months.

I intend to buy a small flat (no mortgage) as an investment and let my daughter (age 30, no partner, no children) live there in the meantime while she is saving up for a deposit on a house, with the intention that she would just pay me a small sum every month to just cover the cost of the service charge/ground rent on the new flat.

She won't have a tenancy agreement (not needed as I won't have a mortgage).

Are there any tax implications or illegality receiving a small cash monthly sum from her? I can't seem to find anything on the internet about this.

Any advice greatly received!

I have a vague memory about there being tax rules along the lines of letting to a relative at a preferential rate but tax being applied as if letting at market rate. So I'd do some more ferreting around the income tax regulations to check.

Thank you, good advice!

I don't know if I'm being naive here, and if someone could explain as I'm clueless, but I can't see how they would know. My daughter would be staying in my house informally, no tenancy agreement, and she me a small amount of cash now and again. 🤔