Transferring properties within family

[jennifer123]

I currently own a flat in Central London for which my parents gifted me the money to purchase approx 8 years ago. I am the only person on the deeds and have lived here throughout that time.

Earlier this year my parents gifted me more money to purchase another house which will now become my primary residence. In order to even up the gifts from my parents I would like to give 40% of the flat to my sister.

1. Am I correct in thinking that I can add her to the deeds of my current flat without incurring stamp duty?
2. Are there any disadvantages to the adding of her to the deeds (I realise it will mean we both have to pay the increased stamp duty on any further property purchases).

Finally we will then be renting out the flat. Initially we will rent it to a cousin for mates rates but eventually it is likely to become a commercially let flat. Does this affect any of the above?

Many thanks,

Jennifer