Property Title Transfer - Unmarried Couple

I am the sole owner of a property that I bought in 2005 for ~£140k. My partner moved in with me in 2006, we are unmarried, and the house remains our only property. In 2021 my partner paid off the remaining balance of the mortgage off (~£20k), as I had paid the the rest of the mortgage without her contribution. I wish to transfer the registered title of the property so we are co-owners as joint tenants. The house is currently valued at ~£275k. Are there any tax implications for this scenario?

Hi,
There are no tax implications on the transfer of assets between spouses and civi partners.  
As you are unmarried, there may be a capital gain arising on the transfer of the share to your partner.  
If this property is your main residence, then you would have private residence relief to set against any gain, which may cover the whole gain, meaning there is no tax to pay.  
Please have a look at helpsheet HS283 Private Residence Relief (2023)
for private residence relief and https:
Capital Gains Tax which includes a calculator to help you work out any gain.
Thankyou.