I’ve been flipping for 20 years and I often look at both options on a project but for me, I’ve always renovated and extended as it’s a faster option. My rule of thumb is always, buy below market value first, that’s where you make most of your profit. Then the amount of addition square footage you add must add a minimum of total cost of the renovation. Then break the ceiling price for the area/street. All my houses are high spec and incorporate a/b rated Epc and eco tech as selling points.
good luck