• You have a valid legitimate reason to request for visa extension, under the visa category.
• You were lawfully admitted into the United States with a non-immigrant visa
• Your non-immigrant US visa status remained valid
• You have not committed any crimes that make you ineligible for a visa
• You have not violated the conditions of your admission to USA
• Your passport is valid and will remain valid for the duration of your stay.
• You have definite plans to leave US at the end of the proposed visa extension period.
• Proper evidence for financial support is provided.

You may not apply to extend your stay if you were admitted to the United States in the following categories:

• Crew member (D non-immigrant visa)
• In transit through the United States (C non-immigrant visa)
• In transit through the United States without a visa (TWOV)
• Visa Waiver program
• Fiancé of a U.S. citizen or dependent of a fiancé (K non-immigrant visa)

• Completed I-539 FORM (PDF: https://www.uscis.gov/sites/default/files/document/forms/i-539.pdf Or File Online (https://www.uscis.gov/i539online)

• An application stating the reason for the requested extension.
• Proof of financial support during this extended stay.
• Copy of your return tickets as a proof of your temporary stay intentions.
• Visitor visa (B1-B2) extension fee is $370. If you are applying for spouse and children, they are included in the same fee. There may be a biometric fee of $85 which can vary based on the type of visa you are on.
• Copy of I-94 of each applicant. (Do not send the original I-94)

USCIS recommends that you apply to extend your stay at least 45 days before your authorized stay expires, but the USCIS Service Center must receive your application by the day your authorized stay expires.

Once you file the visa extension application, USCIS will send you a receipt with a receipt number. This is your case number. The approximate processing time will be indicated on the receipt.

You will also be given a biometrics appointment at your closest ASC in order to fingerprinted. This is applicable for the primary applicant as well as all co-dependents irrespective of age, including minors.

There is a $85 biometrics fee applicable to all applicants including minors.

You may be allowed to stay in the US for 240 days after the expiration date on your I-94, provided you have filed for extension of your stay before your I-94 expired and your application is still under review.

You can check your case status online using case/receipt number. https://egov.uscis.gov/casestatus/landing.do

Or call National Customer Service Center 1-800-375-5283

If your application for an extension is approved, you will be issued a replacement I-94 with a new departure date. Make a copy of this approval letter and I-94 and keep into your record, these will be useful for future entry to USA. You should carry these when you make next US trip or appear for a fresh US visa next time.

You can remain in USA up to until this new I-94 date. When you leave USA, must submit both (old and new) I-94 to the airline staff on check in counter.

If your visa extension application is rejected or denied, you will receive a letter that will tell you why the application was denied. You will then be required to leave the USA immediately.

• If you are a multiple entry US visa holder and overstayed your multiple entries your visa may become Void under rule INA 222(g/2).
• You may not be allowed to enter the USA at the port of entry.
• You may be deported if not leave on time.

 Since the time taken for the approval is not known, the best thing a person should do is to keep the travel plan ready based on the original I-94 dates if you get the approval it’s well and good, else should leave the country. This way you keep your option to enter the USA in the future and avoid any legal problems.

No matter what is the result of the extension request, you should always keep copy and proof of all paperwork and communication you did with USCIS this will be useful for your future visa need for future US trips.

The CIS generally allows you 30 days to depart the U.S. starting from the date on the letter notifying you of their decision to deny an extension. If you do not depart within 30 days, you will be considered deportable. The CIS cautions that if you are refused permission to extend your stay, you may encounter problems with Consulates overseas the next time you apply for a U.S. visa because their computer records will indicate that you did not leave the U.S. within the time frame of your initial period of entry. Be sure to keep your rejection letter and proof of the date of your departure (a boarding pass is the best thing, but passport stamps showing entry into another country is also helpful) to give the consulate the next time you apply for a new visa. Having those may mitigate your apparent overstay and could improve your chances of renewing your visa without the five year restriction usually applied to people that have overstayed their visit.
Source : US Customs official website 

If USICS receive your application before your status expires (or, in exceptional cases, we excuse filing after your status expires due to circumstances beyond your control), and if you have not violated the terms of your status and meet the basic eligibility requirements, then you may continue your previously approved activities in the U.S.( including previously authorized work, for a period of up to 240 days), until we make a decision on your application or until the reason for your requested extension has been accomplished- whichever comes first.
[ Source : https://www.uscis.gov/ ]

If you leave the US before a decision is made on your application to extend and you plan to return to the U.S. in the future, please keep a copy of your application plus the receipt notice to show to the Immigration Inspector on your return travel to the U.S. Otherwise, you may be denied entry for overstaying on your last visit.
[ Source: https://www.uscis.gov/ ]