Transferring house between siblings

My son has owned a house for the past four years and was purchased by way of a loan from me.
What would the costs be if he was to transfer this house with no money changing hands to his sister who would ultimately be responsible to pay me back the loan when the house got sold ? Would there be a requirement for two solicitors ?

2 solicitors required. Daughter will pay stamp duty at 1% of market value. Some other smaller outlays, such as land registry.

If he hasn't used house as his PPR there might be capital gains tax for him to pay.

Thanks. House was used as PPR.