I am under 50 and from Norway. I got 30 days when I entered Thailand 23 days ago. To get 30 new days I was wondering if it is possible to take a flight to Singapore, then go back the same day and get 30 new days. Does it work like that or are there some rules I do not know about? I don't work here so I am just here as a tourist for 87 days.

AFIK by flight it's ok and you'll get another 30 days, you can only do this a limited number of times though I believe, max 3 times I think.

If by land you will get 15 days upon reentry.

If entering on a visa exemption on arrival (as most people do) you may be asked to show an onward flight.

30 days by air and 15 days by land.

There are no limits but you may be questioned after a few land entries.

As you seem to require 57 more days suggest you visit a Consulate overnight to obtain a tourist visa for a 60 day entry. You can do this by land if you want. Take your home flight ticket and hotel booking proof as Consulate might ask for them in local area. If any problem you can just return by air for the 30 day entry you are planning now.

Can you still get a 7 day extension at local immigration on a 30 day visa on arrival?

Can you still get a 7 day extension at local immigration on a 30 day visa on arrival?

Yes

Done it.

I come and go all the time by air - but I have never returned on the same day. If you just need another 30 days suggest you do as already posted and get the double entry tourist visa.

Never been questioned about how many 30 days stamps I have. Only once asked if I had flght booked to leave and I had not. But just waved through any way with another 30 days.

The 7 days is an under review for extension that is disapproved rather than an official extension (but yes it will allow 7 more days). It will normally be issued on any type of stay to allow a late departure but may require show of tickets (and the normal 1,900 baht application fee).

Poster does not need a double entry tourist visa. Single or two 30 day entries is all he needs as he will have already stayed 30 of the 87 total days.