166% price increase in a year

[Bells2323]

https://www.rightmove.co.uk/properties/142383209#/?channel=RES_BUY

what must the owners have done for the house to increase in value?

It says 'completely remodelled' so perhaps it was a total wreck or HMO and they've transformed it. If I had that much, I wouldn't live there but it is nicely done up and SN is a desirable place for lots of people.