Bluetoon

I’ve owned my house for over 30 years and it has been my sole residence for the whole of that period. I’m planning to sell this (‘old’) house and purchase another (‘new’) house, which will become my sole residence. My query relates to the scenario where I’m unable to sell my old house at the same time as I purchase my new house. I understand from HMRC guidance that private residence relief would only apply if, having moved into my new house, I sell my old house within 9 months of completing the purchase of my new house. Is my understanding correct?