Statistically the biggest health and safety risk is the water feature. If you have a pool you need some-one who stays sober the entire evening. If the water feature has a cover on it, then it needs to be taken off (some-one could fall under it).
Fire risk from candles comes a close second. Do you know where the fire extinguishers are located? There are many battery operated authentic alternatives to burning candles these days.
Trip hazards e.g. stairs with open risers. Fine when you are sober, but not good when you are unfamiliar with the surroundings and a couple of drinks down.
Have a spillage kit at the ready – red wine is the killer. Salt, soda water and a bucket or towel to put over the stain area so guests avoid it.
Toilets – ideally you need a dedicated his and hers! Remember to place a spare toilet roll by the WC.
Throwing cigarettes over the balcony and glasses over or on the balcony. If you use glass then make sure you have insurance. Perhaps use plastic cups on the balcony?
Have a policy to deal with unwanted guests. The hostess to challenge accompanied by two burly guests?
Pets and legs and other prized possessions – Keep them safely out of the way, otherwise someone might accidentally step or sit on your prized animal. Equally store that Ming vase out of harm’s way.
Beware overloading your electrical system – consider what you are plugging in, if you have a DJ have you got enough power?
Noise pollution – you don’t want the police to show up unannounced. See the HK Government’s “Concise Guide to Noise Control Ordinance”. Under section 4 of the ordinance, violating the general noise control between 11pm and 7am or on a general holiday invokes a fine up to HK$10,000.
At the end of the evening have you accounted for everybody? Or is a stray guest slumped in a corner? Make sure they are ok even if it means waking them up. They may be with you for the night……