Hello and welcome to JustAnswer. I am here to help you. I am reviewing your question and will respond to you shortly.Many thanks
Thank you for your question.
You are right is your understanding and thought process.
I doubt properties would keep increasing in value year on year over the next 2 years.
Having said that, if you were to hold on to the property until Dec 2018 and it is let, although you would have surpassed the period allowed for additional relief (final 18 months of ownership), there is a further relief available called letting relief and the maximum relief available is £40k.
Bear in mind there is gains annual allowance to be taken into account before you working out chargeable gain on which CGT is payable.
More information on private residence relief including letting relief can be found here
I hope this is helpful and answers your question.
If you have any other questions, please ask me before you rate my service – I’ll be happy to respond.