From London US Embassy re additional US passport pages
Extra Pages
If your passport's visa pages are becoming full, but your passport is still valid and in good condition, you may apply by mail or courier for additional pages to be added to your passport. Only one set of extra pages (24 pages are in each set) can be added to your passport at a time, but if needed, and if your passport remains in good condition, you may have up to three sets of extra pages inserted into your passport. Once this limit is reached, you will need to apply for passport renewal. Please note that extra pages cannot be added to an emergency passport.
Application Requirements
Please submit the following by Royal Mail Special Delivery or via the Embassy's appointed courier service:
* Your valid ten- or five-year passport;
* Passport form DS-4085: Application for Additional Visa Pages. Please ignore page one of the printed instruction pages, as it does not apply when applying for extra pages in the United Kingdom. Follow the instructions on this page. If you experience trouble with the Passport Application Wizard, click here for Form DS-4085 [PDF]. For tips on completing the DS-4085 [PDF], follow this link;
* Fees: no fee is charged for this service.