It says the changes may have contributed to the March 8 blaze at the hotel which killed two people.
City officials in Klong Toey district where the hotel is located, are compiling evidence with a view to laying charges.
They say the hotel remodelled a portion of its fifth-floor car park into a banquet room despite its low ceiling, and also failed to install sprinklers.
On the night of March 8, smoke from a fire which started in the banquet room spread quickly through the hotel’s ventilation system.
Two Russian guests succumbed to smoke inhalation and 22 others were treated for minor injuries.
Phinit Loetudomthana, of the Public Works Department, said he will advise the Klong Toey district office to file both civil and criminal lawsuits against the hotel owner.
Violation of the 1992 Building Code Act carries a three-month jail sentence and a fine of up to 60,000 baht.
Deputy Bangkok Governor Teerachon Manomaiphibul said the Klong Toey district office could not be blamed for failing to carry out an inspection earlier.
The responsibility clearly lay with the owner, who failed to inform the BMA and changed the layout of an area to accommodate something other than the purpose for which it was built, Mr Teerachon said.
The fire at the 221-room hotel on Sukhumvit Soi 22 occurred just five days after a blaze on March 3 destroyed the 13-storey Fico Place building on Sukhumvit Soi 21.
This has alerted the BMA to the need for better safety precautions in high-rises and old buildings.
Bangkok Governor MR Sukhumbhand Paribatra said the city is compiling a database of these buildings and will set up panels in each of the 50 districts to start inspecting the buildings from Thursday.
The BMA has come up with a six-point measure to ensure all city buildings comply with safety laws. Building owners will now be required to allow authorities to examine the legality of documents they used in applying for construction permits.
Owners who have not had their buildings checked for safety by experts will be asked to hire private inspectors certified by the Department of Public Works and Town & Country, to conduct the inspection and file a report with the department.
Building owners showing non-compliance will face legal action from the BMA.