When you were granted your construction permit, you were given 18 months to construct your station.  The rules do permit you to have up to three years if necessary.  Based on information received from FCC Audio Divsion staff, this is the process you need to go through to obtain an extension on your construction permit:

First of all, you will need to wait until two months prior to the expiration of your construction permit.  If you need to determine your expiration date, please visit fccdata.org from REC and look up your facility. 

Second, you must have good cause for the extension. There must be a good reason why you could not get the station constructed in 18 months. The FCC not provided any guidace on what is considered good cause. 

Third, you can make the request an informal request.  In other words, you will not file an application or make another electronic filing.  Instead, you will need to send the request in triplicate to the Office Of The Secretary:

If you plan to use first class mail, USPS Priority Mail or USPS Express Mail, send to this address:

Marlene H. Dortch, Secretary
Federal Communications Commission
Office of the Secretary
445 12th Street, SW
Washington, DC 20554

If you plan to use UPS, FedEx, DHL, etc., send to this address:

Marlene H. Dortch, Secretary
Federal Communications Commission
Office of the Secretary
9300 East Hampton Drive
Capitol Heights, MD 20743

Fourth, you will need to send a courtesy copy via e-mail to one of the following FCC staff members based on the situation:

• For LPFM construction permit holders who have been granted their 18-month CPs and are routinely requesting to extend it to 36-months as allowed by the rules, send a courtesy copy of your request to Gary Loehrs (gary dot loehrs [[at]] F C C dot gov).
• For anything other than the routine 18 to 36 month request, please send a courtesy copy to Victoria McCauley (victoria dot mccauley [[at]] F C C dot gov). 

Applicants with a Mexican concurrence condition:

If your construction permit contains a condition statement like this:

This application is being granted prior to the completion of the international notification process. Therefore, any construction of and operation with the facilities specified herein is at applicant's own risk and subject to modification, suspension or termination without right to hearing, if objected to by the Instituto Federal de Telecomunicaciones ("IFT") in Mexico or if found by the Commission or the IFT to be necessary in order to conform to the 1992 USA-Mexico FM Broadcasting Agreement. This condition will be removed if formal acceptance of the facilities granted herein is received from IFT.

your construction permit is immediately eligible for tolling for the period that it takes for Mexican concurrence.  Once Mexican concurrence is received, then that time can be added to the construction permit.  In other words, if you wish to wait for Mexican concurrence prior to constructing you can do so.