I have a single entry non-immigrant B visa, which is valid for a 3 months stay.

On the visa it says "Extension of stay not permitted." The sentence has beed crossed off and there is a second signature next to this sentence.

My question is: Can this visa be extended? Where in BKK and for how long?

I have a single entry non-immigrant B visa, which is valid for a 3 months stay.

On the visa it says "Extension of stay not permitted." The sentence has beed crossed off and there is a second signature next to this sentence.

My question is: Can this visa be extended? Where in BKK and for how long?

BKK immigration.

Extend for up to one year depending upon the validity period of your work permit (assuming you have one).

Non-B stays are not normally extendable under other circumstances.

As said it can be extended for work but the period would be one year from current entry and then the work permit would follow the visa time. You would also have to meet pay minimum requirements for your nationality.