The legal requirements for the health and safety of individuals in a building open to the public are fully outlined at your local town, city and/or county offices, and are very localized in nature; one answer will not suffice for all locations. Relevant rules and regulations may also be outlined at the state and federal (US) level and the Home Office (UK) when it comes to disability access and potentially hazardous conditions in a building.
Start with your local city/county office and local fire department websites; many municipal sites will give you the standards for occupancy numbers, handicapped access, fire exits, alarms and egress, and building safety codes, and have application forms. You may have to make personal or organization board representative visits to each office. The authorities may require on-site inspections of your building.
The city/county can also advise you on who to talk to in order to comply with any relevant federal or national standards; or look for those websites.
This nothing to ignore; your charity, be it a 501(c)(3) (US) or a Charity (UK), and in some cases, the people on the board, has considerable liability if you own a building and are open to the public without code fire exits, alarms, smoke detectors, or occupancy standards and there are injuries or deaths that would be attributable to the lack of fire and building inspections and licenses.
Even if there are no incidents, if you don't have necessary licenses and inspections, you can be liable for fines.