Find out what the best house on the street would sell for, as this will give you a guide as to the maximum amount you should spend. For example, if the most expensive property in your area would sell for THB 6,000,000 and you bought your property for THB 4,500,000, cap your renovation budget at THB 1,500,000. If you are considering extending, then make sure it is not more financially viable to sell your property and upgrade to a bigger property in a more desirable location. Also, there is little point in renovating if the basic structure of the property is unsound. The renovations that are most likely to improve your resale value are those done to improve the look and function of kitchens, bathrooms and entertainment areas.
Landscaping is also a wise investment.
If you are considering renovating in order to sell your house in the immediate future, be extra careful. Renovations will need to be made which suit the tastes of prospective buyers, and these may not necessarily be the same as yours. You need to put yourself in the buyer's shoes. One trick is to stand across the street from your property and imagine that you are a potential buyer, viewing the property for the first time. Any renovations you make to improve the chance of a buyer wishing to inspect the property further could be handsomely rewarded.