Requesting a deadline extension

In some cases you may need more time than the employer has allowed to make a decision. In that case:

You may ask for an extension. The employer does not have to grant it.
Make sure you have a concrete and appropriate reason for asking for an extension. Expecting to hear soon from another employer with whom you've interviewed is a legitimate reason. If you are just hoping to get more interviews, that is not a concrete reason.
Don’t wait until the last minute to ask for an extension. This looks like you don't think ahead and may signal that's how you'll behave on the job.
Be tactful and diplomatic in your wording. You will need to explain your reasons to the employer. For example, if you have an upcoming, previously-scheduled interview with another employer, you may explain that it is important to you to keep your commitment to the other employer, and that in order to make the best decision, you need to attend the other interview.
For the sake of speed, phone the employer to discuss the situation. For the record, follow up in writing. Since this is a request that needs to be handled quickly, use email to confirm your request in writing. See the sample letter below. (If the employer granted your request verbally, use the email to confirm that, and thank the employer.)
Be familiar with guidance provided to employers on providing a reasonable amount of time for a job candidate to make a decision. NACE, the National Association of Colleges and Employers, provides an Advisory Opinion on Setting Reasonable Deadlines for Job Offers.
If you still have questions, please seeking advising. We're here to help you.

Sample letter:

Dr. Thomas G. Jones
Oregon Department of Fish and Wildlife
2300 Main Street
Portland, OR 90001
tgjonesemailaddress@agency.gov

Thank you for your telephone call and letter that I received yesterday offering me the position of wildlife biologist with the Oregon Department of Fish and Wildlife. I am excited about the opportunity this position offers both in terms of job duties and location, and I very much appreciate your confidence in offering me the job.

You asked that I make a decision by next week on March 7. This is an important decision, and I am in a situation where I will not have all the information I need in order to make this decision by that date. To confirm our phone conversation of this afternoon, I am asking you to consider whether it would be possible for me to supply you with my decision by March 28. I would very much appreciate such an extension, and assure you that I will be able to make a firm decision by that date.

Thank you very much for your consideration of my request.

Sincerely,
Hannah Lenke
Fish and Wildlife Sciences and Biology double-major | Virginia Tech
343 Jefferson Street
Blacksburg, VA 24060
(540) 555-8754
HannahLenke@vt.edu