Due diligence in the real estate industry is basically investigating a property to help ensure everything is as it is presented to be. The experts would look into the property for any unforeseen issues that might be a problem if you went ahead with the investment.
This helps to protect you against people who are being dishonest, and against genuine oversights. It is usually a relatively straightforward process because it will be carried out by the experts, especially your real estate agent and your property lawyer. It might sometimes be a little time-consuming, and there is a cost, but it saves you a lot of money and inconveniences further down the line.
Anybody looking to invest in Thailand or in Koh Samui, should undertake their own due diligence. Most people will not know the Thai property market well enough to do it themselves, and then there is the language barrier to consider. However, there are services available that will perform due diligence in Thailand for you.
Depending on the property in particular, the real estate agent and/or the lawyer will also undertake some degree of due diligence for you. However, it should be noted that this is not mandatory by Thai law. Rather, it is simply a matter of additional security that investors should pay attention to so they can be sure their investment is safe.
Serious problems are uncommon, so you are unlikely to have any unpleasant surprises if you do go ahead without due diligence in Thailand. Considering the potential risks of not doing so, however, means it is advisable to be on the safe side.