no no payment but I plan to pay for the newbuild property to incorporate an annex for my mother on her land
Your question is concerning tax liabilities if the aforementioned scenarios take place
As you would become a joint owner of land/property and the title deed would reflect it, any gain made on transfer of interest to you by your mother albeit chargeable to CGT would be covered by private residence relief for her CGT purposes.
If there is no consideration, then the value of your share would be deemed a gift as a potentially exempt transfer for IHT purposes as far as your mother is concerned. The seven year rule applies to PETs..if she survives for 7 years after making the gift, the gift is out of scope for IHT purposes..i.e, tax exempt.
More information on gifts and implication on IHT can be found here
Any gain you make from selling your property to fund this project would be covered by private residence relief as it is your main residence.
If you were to opt for external financing for this project and retain your own property for letting purposes, then you would qualify for PRR partially, to cover the period the property was your main residence as opposed to total period of ownership.
If the property is let during that time, you would be able to claim letting relief for period let. The final 18 months of ownership always qualifies provided the property has been your main residence at some point during period of ownership.
More information on private residence relief is covered in HMRC helpsheet hs283 here (please look at how the relief works) https://www.gov.uk/government/publications/private-residence-relief-hs283-self-assessment-helpsheet/hs283-private-residence-relief
I hope this is helpful and answers your question.
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