I am looking for an idiom to describe an unintended benefit that results due to an action taken.
Not an idiom, but a single word could be...
- something advantageous or valuable that is received or obtained as a benefit beyond what is due or expected. (Dictionary.com)
To cite an example, I'm reminded of a line of Charlie's from 2½ Men:
"Alan, he was happy to see his friends. Being away from you is just gravy." (=unintended benefit)
If the benefit is financial, you could also use...
an amount of money that somebody/something wins or receives unexpectedly (ODO)
EDIT: Like @Tim said, this can also be used metaphorically to refer to gains other than financial, but it doesn't quite convey the 'unintended' part.
If you're looking for an idiom, rather than a single word, consider:
But, again, these don't quite cover the 'unintended result of your own action'.
Happy Accident describes your intention:
a pleasant situation or event that is not planned or intended - We never planned to have a third child - it was a happy accident
Fortuitous is a somewhat looser fit
adjective 1. happening or produced by chance; accidental: a fortuitous encounter. 2. lucky; fortunate: a series of fortuitous events that advanced her career.
You could call it a side benefit.
Occurring or discovered by chance in a happy or beneficial way
also a stroke of fortune
something good that happens to you when you do not expect it
To walk in and get a job like that was an incredible stroke of luck.
Cambridge Dictionary of American Idioms
Stroke was derived from the verb strike:
"act of striking," c. 1300,
probably from Old English *strac "stroke," from Proto-Germanic *straik- (cognates: Middle Low German strek, German streich, Gothic striks "stroke"); see stroke (v.).
The meaning "mark of a pen" is from 1560s;
that of "a striking of a clock" is from mid-15c.
Sense of "feat, achievement" (as in stroke of luck, 1853) first found 1670s;
the meaning "single pull of an oar or single movement of machinery" is from 1731.
Meaning "apoplectic seizure" is from 1590s (originally the Stroke of God's Hand).
Swimming sense is from 1800.
etymonline.com emphasis mine
A “fluke” is something that happens unexpectedly because of an accident or good luck and could describe such an unintended benefit. Although they’re not always beneficial, “flukes” are usually good things.
“Flukes” can result from a single action (as your question seems to require), as Jed Clampett’s discovery of oil (arguably a benefit to Mr. Clampett) while shooting at a rabbit was a fluke, but they can also be the result of a series of actions or circumstances, or even inaction.
Regarding the distinction between “accidental,” “chance,” “unexpected,” and “unintentional” these words are related as are“unintended, unexpected, and ‘fluke’.
For an idiom using “fluke” for such benefits (and especially the action of obtaining them), you could combine it with “into” or ”upon” to describe “something that was just fluked into or fluked upon,” although these are not nearly as common as other “into”/”unto” idioms such as “fell upon” and “lucked into.”
Single word-> bonanza..... a noun that means "a sudden rush of wealth or good fortune."
- "the demand for testing has created a boom for those unregulated laboratories where boxes of specimen jars are processed like an assembly line"
unexpected help or comfort. (A biblical reference- TFD.)
- It was like manna from heaven for the exercise-haters, for a sedentary society with elevated levels of obesity and diabetes: exercise less and live longer! (Forbes, Feb 8, 2015)
This would only be used in the context of the other results of the action being negative, but the idiom every cloud has a silver lining, or just the phrase silver lining might work.
adjective adjective: serendipitous
occurring or discovered by chance in a happy or beneficial way. "a serendipitous encounter"