What documents are required for a 30-day visa extension in Thailand while staying with a girlfriend?

All I had was passport ,photo, proof of residence i was staying at air bnb she filled out online and I took a copy also and 1800 baht

Listen to what Tod Daniels tells you, he knows his stuff.

One thing I will add is that you should inform your immigration office where you are staying within 24 hours of your arrival in thailand.

The first time I came over I did not do that, and when I went in for the extension I was questioned as to why I had not informed them. I got the extension ok but they said if I come again I must do it.

That will be the same as the TM30 that Tod mentioned I assume.

𝗪𝗵𝗮𝘁 𝗱𝗼𝗰𝘂𝗺𝗲𝗻𝘁𝘀 𝗱𝗼 𝗜 𝗻𝗲𝗲𝗱 𝗳𝗼𝗿 𝗺𝗲 𝘁𝗼 𝘁𝗮𝗸𝗲. 𝗜'𝗺 𝘀𝘁𝗮𝘆𝗶𝗻𝗴 𝗮𝘁 𝗴𝗶𝗿𝗹𝗳𝗿𝗶𝗲𝗻𝗱 𝗵𝗼𝗺𝗲.

This graphic is a list of the most requested documents at immigration offices for a 30 day extension based on tourism. It is NOT office specific so your office may or may not require everything on the list

𝗗𝗼𝗲𝘀 𝘀𝗵𝗲 𝗻𝗲𝗲𝗱 𝘁𝗼 𝘁𝗮𝗸𝗲 𝗱𝗼𝗰𝘂𝗺𝗲𝗻𝘁𝘀 𝗮𝗹𝘀𝗼 𝘁𝗼 𝘀𝗵𝗼𝘄 𝘄𝗵𝗲𝗿𝗲 𝗜'𝗺 𝘀𝘁𝗮𝘆𝗶𝗻𝗴

She needs to file a TM30 (notification of foreigner in residence) form either in person at the immigration office or online at this website. Many offices require this to be filed before they will let you get an extension.

Take her and her house book copy with you when you go for the extension and file when you're there

𝗖𝗮𝗻 𝗜 𝗴𝗲𝘁 𝗺𝘆 𝗻𝗮𝗺𝗲 𝗽𝘂𝘁 𝗼𝗻 𝗵𝗲𝗿 𝗵𝗼𝗺𝗲 𝗯𝗼𝗼𝗸 .

Sorry not a visa question at all, you both should go to the local Amphur (City Hall) and ask about the process