Alistair bought a house on 1 April 1997 for £125,000 and occupied the entire house as his principal private residence until 1 November 2005. As from that date, he rented the top floor (comprising one -half of the house) to ten ants and retained the ground floor as his own residence. This arrangement continued until 1 October 2017 when he sold the house for £615,000. Compute the chargeable gain.