Some Laws for Writing a Detective Story

1. A reader must have the same opportunity with the investigator for solving the crime.  All the clues must be plainly stated and reported.
2. No deliberate tricks or dishonesty may be placed on a reader other than those performed legitimately by the perpetrator on the detective.
3. There must be no love engagement.  The profession in hand is to bring a perpetrator to the bar of justice, but not to bring a lovelorn pair to their wedding
 day.
4. The detective themselves, or one of the official police investigators, should never turn out to be the offender.  This is plain trickery, on a par with offering someone a penny for a ten-pound note.  It’s false pretences.
5. The offender must be concluded by logical reasoning — not by accident or coincidence or unmotivated admission.
6. A detective novel must have a detective in it, and a detective is not a detective unless he or she detects. Their purpose is to collect clues that will ultimately lead to the person who did the dirty work in the first chapter being apprehended. If the detective does not reach their conclusions through an investigation of those clues, he has no more solved the problem than the pupil who gets their answer out of the back of the arithmetic textbook.
7. There frankly must be a corpse in a detective novel. No lesser crime than murder will suffice.  A couple of hundred pages is far too much bother for a crime other than a murder, or a few.  After all, the reader’s task and investment of energy must be rewarded.
8. The puzzle of the crime must be solved by naturalistic means. Such methods for learning the truth as slate-writing, ouija-boards, mind-reading, spiritualistic seances,  crystal-gazing, and the like, are taboo.  The reader has a chance when matching his wits with a typical detective, but if they must compete with the world of ghosts and go chasing around metaphysics, they are surely defeated.
9. There must be only one detective — that is, but one protagonist.
10. The offender must turn out to be a person who has acted a more or less prominent part in the story (antagonist) that is, a character with whom the reader is familiar and in whom they take an interest.
11. The butler or servant should not be made by the author as the culprit. This is begging the question; it is a too easy solution.  The culprit must be a decidedly worth-while person — one that would not ordinarily come under suspicion.
12. There must only be but one offender, no matter how many murders are perpetrated.

Remove unnecessary words

On a different note to my regular diatribe, passive voice can also make your writing more concise, strangely enough, by removing unnecessary words, i.e. information that isn’t required to get the message across or are unimportant.
“John Doe was hit by a car and then rushed to hospital.”
“John Doe and what happened to him is the focus of this sentence, not the thing that hit him.”
Additionally, it isn’t important how he got to a hospital, only that he’s there. (I’m particularly upset at ending a sentence with a preposition) 
This passive construction is much more efficient than;
“A car hit John Doe, and then an ambulance rushed him to a hospital,” which sounds as awkward as ‘passive’ sounds when ‘active’ should be used.
It isn’t vague like passive can be, but it isn’t concise, either. Another example is “John Doe was charged with the murder of his wife, Jane,” which sounds much simpler than “Police charged John Doe with the murder of his wife, Jane,” or even “Sergeant James Ryan charged John Doe with the murder of his wife, Jane,” where it isn’t clear whose wife Jane is. (or this one)

If writing a short story that only should contain 1500 words then you could look for the words that just add completeness like;

“John sat down and read a book.”  –  6 words

“John sat and read a book.”  –  5 words

There is sat down and stud up.