Unless you are in a Conservation area it’s doubtful you will need planning permission as long as all the Utilities are connected to and paid for by the main dwelling, the building is occupied by family members only and that the properties cannot be sold separately. Building regulations will have to be applied.
We looked into this very thoroughly during Covid, I will look at MrOops iPad later and find the Company name, they deal with everything, the slab, connecting utilities and total build a 2 bed, 2 bath home back in 2020 was £150,000, they look beautiful but we didn’t go as far as going to see one as our daughter didn’t want to live at the bottom of our garden ( it’s a large garden so plenty of room). The one we looked at was 60’x40’. Approx.
However, divorce could really leave you homeless, as your annexe will be tied to your daughters house and unable to be detached and sold as a separate dwelling.
The company we looked into will also do check to make sure your particular council allows an annexe on your property.