How old is the property: Age of the property determine whether to renovate or build from the scratch. If it too old and there is a no option of remodeling then it is time to construct a new one. But if the property is not that old and has been newly constructed then small renovation can work better and could be affordable too.
Decide your budget: this is the most important aspect, as a lot of things depends on budget. You need to think about your capabilities and how much you can afford as whether you built from scratch or renovate both needs a good amount of money. So in case of renovation check which areas need to renovate and how much. If renovation is too much like almost the entire house then building from the scratch is a better option,
Growing requirement: with time lifestyle also changes. Add on family members, children often calls for a bigger space such an extra room or a bigger garden. So you have to decide whether with just a remodeling you will get a bigger space within your existing property or you need to build a new one. For instance, your brother is getting married that means now he will have his own family so in this case, you will need a separate house or a separate floor hence you need to construct a new one. But if you to increase space of a room then renovation is enough.
Post construction or renovation cost: spending money while renovation or construction is not the only time when money is required. But after construction and renovation also you might need to spend some extra amount. If you have constructed from scratch then the maintenance cost might be less but in case of renovation, you might need to spend a good amount to maintain that new equipment or to rectify continuous blockages.
Thus make sure to have a look at all these factors and then decide upon renovation and constructing a new one.