Only use a single space if the teacher asks explicitly for it.
That said, the few colleges that do specify a preference typically request double-spacing.
Also, if you read the blogs and FAQs written by college admissions officers, you'll usually find a general preference for double-spacing.
Another sample block-style letter is provided below from the e Book Everyday Letters for Busy People.
(Click on the link to the left or the image below to go directly to this section in the e Book!
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In text formatting, a double space means sentences contain a full blank line (the equivalent of the full height of a line of text) between the rows of words.Today, with bigger hard drives and newer file systems Double Space is no longer used.Double spacing refers to the amount of space that shows between the individual lines of your paper.Applicants used to be able to attach their essay to the application, a feature that required the writer to make all kinds of decisions about formatting.The most recent version of the Common Application, however, requires you to enter the essay into a text box, and you won't have any spacing options.If the application provides formatting guidelines, you should obviously follow them. An applicant who can't follow directions on the application is someone who is likely to have problems following directions on college assignments. So if a school says to double space with a 12-point Times Roman font, show that you pay attention to both details and instructions.If the application does not provide style guidelines, the bottom line is that either single- or double-spacing is probably fine.The white space between the lines leaves room for editing marks and comments.Double spacing is the norm for essay assignments, so if you are in doubt about expectations, you should format your paper with double spacing.When a paper is single-spaced, there is very little white space between the typed lines, which means there is no room for marks or comments.This is precisely why teachers ask you to double space.