Building on Koh Mak (and for sure also on any other Thai island that does not have a bridge or ferry connection to mainland) is not an easy task. It will easily cost double the amount. Experience shows even more.

Materials have to be brought either by barge/pontoon or with the wooden transport boat. It is an expensive and time consuming task.

Compare 1 cubic meter of concrete in Bangkok. It will cost in average 1800 Baht depending on the mixture. Up to 2200 for specialized mixes.

1 cubic meter of concrete on Koh Mak is 4500 Baht. There is no specialized mix (more cement for example) unless you are personally present where the concrete is mixed and you control the procedure of every load (and you of course pay the added cement).

Whilst in Bangkok you will have 4 and more cubic meters of concrete in one truck load, here on Koh Mak it will around 0.6 to 1.5. 

Often it will happen that there is no more sand, gravel or cement available. Then all you can do is wait for the next delivery. Can take up to one week. There is no other company that you could order from as an alternative.

Local workers have learned to build the Thai way. This means cement columns and thin walls in between. Building with aerated blocks is only slowly becoming popular. Building without columns is uncommon and you will find it difficult to convince workers to build this way despite having an approved building plan from an architect. 

You will find it difficult to have your workers actually “glue” the lightweight stones rather than applying 3 centimeters of mortar.

So, even if back at home you built your own house with your own hands or if you ran a building company for years: you will probably fail when trying to apply your experience to local customs.

If you have an expensive project you might be able to find an experienced building company from mainland. Also here experience shows that they will do your project, but at the end they will have learned how difficult it is to build here. They very certainly will not be ready to come back for the next project. And the more it happens the more this knowledge spreads and the bigger the chances are you might not find a company that is willing to come from from mainland anymore.