Definitely worth appealing, especially as you bought close to the lowest point of the market and they are now valuing after 3 years of rise. You’ll need a Spanish tasador valuation and a valuer experienced in this should be able to take care of it all for you. It’s valuer against valuer after all. At the very least you’ll force the tax authorities to justify their value. You need to move fast as you have a short time to appeal. For the future, where you are buying at a ‘good’ price it can be best to get a tasador report at or around that value and attach that to the excritura (Title) when its submitted to the Registrar. That way the tax authorities will know they have a fight on their hands if they try for the ‘complementary tax’. Now there’s a misnomer if ever there was one! You’ll find helpful advice on the internet.