You can buy or download many plans that will be approved for building.
The actual permit for my house was 35 baht but that was just over ten years ago.
Maybe double now!!!
Also if you make your own basic plan the architect in the local
amphur will help you to adjust anything that is needed,for a small fee.
I made my own plans but i had a licensed architect make the official drawing including all the materials needed,they can also price it so you know what the building will cost you.
The reason i did this is that if you ever want to sell your house you can show them the whole plan and the material that went into it.
I think that was worth the 20 000 baht i paid him.