I visit Thailand 2-3 times a year, Always on the 30 day visa exempt. I have never been asked for anything at immigration, i Always make sure i have the 40,000 BHT in hard currency, an outward ticket, but never any pre booked accomodation. If im visiting for say 3 weeks, how can i pre book accomodation as i rarely know how long or where im staying?. IE i might stay 2 nights in Bangkok or maybe longer before travelling in Thailand. I am British passport holder.
TLDR : Answer Summary
A British passport holder visiting Thailand on a 30-day visa exempt status is concerned about not having pre-booked accommodation. Many commenters suggest that it's sufficient to book just the first few nights of stay, or even to provide an address of a friend or relative instead. It's generally accepted that proving accommodation for the entire duration of stay is not strictly necessary, as long as you have sufficient funds (around 20,000-40,000 THB) and an outward ticket. Some users report experiences of not being questioned at immigration regarding accommodations or cash.