The Ten Commandments

(or the rules I follow to write Whodunits)

Thou must remember there aren’t actually any commandments. I’m not bound to follow any rules, but…
The murderer shall be known to the reader within the first three chapters. I know I broke this rule in one book, but he wasn’t the uber-villain, so the rules stands.
The astute reader shall be able to work out who did it three quarters of the way through the book. No last chapter clues.
As with ‘Death in Paradise’ there shall only be four or less viable suspects at the denouement. Unlike DiP, many more should be tainted with the whiff of villainy in the earlier chapters.
There shall be sub-plots. These hinder the curious spotting the obvious solution.
The protagonist(s) shall have a story arc. No Hercule Poirots starting as smug smart-arses and remaining so throughout the tale.
Thou shalt not portray any police person as an idiot. Misogynist, racist, arrogant, yes, but they don’t get hired for stupidity.
Thou shalt ignore the anal retentives who point out the police would never carry out an investigation in the way you describe. Your public know better. They’ve seen Morse and Midsomer.
Do not allow Commandment #8 to suggest the constabulary are happy to leave the investigation to priests, nuns, and vicars. They have better things to do.
Keep it under 90,000 words.