From 6 April 2016 there is an additional 3% Stamp Duty Land Tax payable on the purchase of a second or subsequent property.
The additional 3% rate does not apply if you are replacing your main residence, but if the old main residence is not sold by the time you purchase your new property, then you only have 36 months in which to sell.
You can also get caught out if you buy a property following separation from your spouse or civil partner unless you are formally separated.
Be warned too if you are a landlord already and are yourself living in a rented property owned by someone else. If you decide to buy yourself a new property as your main residence you have to suffer the additional 3% Stamp Duty on your purchase.