In 1930 a small group of mainly English and Irish detective fiction writers (including Agatha Christie, Dorothy L.Sayers, E.C.Bentley, Emma Orczy, and G.K. Chesterton formed the Detection Club, a kind of dinner club and guild, based in London, England. The club remains an invitation only organisation, and one of the main caveats is that members adhere to Knox's 10 rules of detective fiction - though the rules are less strict these days, but I think they are of great help to new cozy mystery writers like myself. I hope you will find them useful too.
Detection Club Oath
"Do you promise that your detectives shall well and truly detect the crimes presented to them using those wits which it may please you to bestow upon them and not placing reliance on nor making use of Divine Revelation, Feminine Intuition, Mumbo Jumbo, Jiggery-Pokery, Coincidence, or Act of God?"
Knox's 10 Commandments of Detective Fiction Writing