How common is the hotel forbidding third-party laundry?
Very rare in Bangkok. About 99% of hotels accept third-party laundry pickup (premium chains, boutiques, hostels, business hotels). The 1% that explicitly forbid: usually all-inclusive resort hotels with included laundry packages (forbidding third-party prevents customers from skipping the included service), or some luxury serviced-apartment buildings with strict security policies that don't allow non-resident pickup staff to enter. For 99% of Bangkok tourists, this is a non-issue.