What is the best way to extend my visa in Thailand while staying at a friend's condo without an official rental?

Regarding your visa extension, you’re right that you will need a TM30. Even though you are staying at your friend’s condo, it is their responsibility (or the condo owner’s) to file the TM30 to notify immigration of your stay. Your friend should be able to do this online or in person at the local immigration office. Once the TM30 is filed, you’ll receive a receipt, which you can use for your visa extension.

For the extension itself, you can apply for a 30-day extension at the immigration office for a fee of 1,900 THB.

Hope this helps, and good luck with the process!

I always ask for a copy of the TM30 when checking in, the it will be the least extra work from the hotel, and you suggests that you may need it later

Just check into a hotel for a night, check they'll give you a TM30 first. Then just take a copy of the TM30 from the hotel and apply for the extension. Don't worry about not having filed one previously

For TM30 the property owner is legally required to file for you if you are officially residing there, and if the unit is rented by your friend, will have to seek assistance from landlord to file the TM30.

The easiest way for you would be to book a cheap hostel for 2-3 days (even if you don't stay there) and ask the hostel manager to fill TM30 on your behalf and give you the TM30 receipt.

However, there's a catch. If you didn't do TM30 within 3 days of your arrival in Thailand, the hostel manager cannot file TM30 on your behalf. That's because you'll first need to pay a fine of 1,600 THB for not filing TM30 upon your arrival in Thailand. You'll need to visit your nearest Immigration Office with the documents that the required documents from the Hostel manager. First you'll pay the fine, then you'll file the TM30 form and get the receipt. Then you can apply for extension.

If the house owner will not file TM30 you need to contact Immigration Office for advice. You won't get an extention if you are not registered at an address via TM30

You need to go to an Immigration Office for your extension and the price is 1900 thb everywhere.

You friend or their house owner can file a TM30 for you online. By far the easiest way to do it.