The Special Enrollment Period for Medicare Part B is one of the most important safety nets in the federal retirement system, and one of the most often misunderstood. Federal employees who continue working past 65 and stay on FEHB — rather than enrolling in Part B at 65 — rely on the SEP to enroll in Part B at retirement without paying the late-enrollment penalty. Used correctly, the SEP turns what looks like a missed deadline into a deliberate, penalty-free enrollment.
Used incorrectly — or relied upon without understanding the rules — the SEP can leave a federal retiree paying a permanent Part B surcharge for the rest of retirement. The difference between the two outcomes is mostly about what counts as creditable coverage during the working years and how promptly the retiree enrolls after that coverage ends.
The basic shape of the SEP.
For a federal employee who is 65 or older and covered by an Employer Group Health Plan (EGHP) through current employment— their own active federal employment or their spouse’s active employment with a large employer — the SEP opens when that employment ends. The SEP is generally eight months long, beginning the month after the active employment or EGHP coverage ends.
During that eight-month window, the retiree can enroll in Part B without a late-enrollment penalty. The eight months is measured from the loss of active employment or loss of EGHP coverage, whichever happens first. The window is not a suggestion. It is the cleanest path the system offers, and it closes on its own clock.
What counts as “current employment”.
The SEP’s most restrictive rule, and the one most frequently misapplied, is the requirement that the EGHP coverage be tied to currentemployment. Federal retiree coverage is not current employment. COBRA coverage is not current employment. Marketplace coverage is not current employment. Coverage as a dependent under a former employer’s retiree plan is not current employment.
The rule is also specific about who counts as an employee. Active federal employees count. Active employees of large private employers (generally 20 or more employees) count. Active state and local government employees count. The line is bright, and it is drawn around the active employment relationship.
The retiree coverage trap.
A federal employee who retires, transitions to retiree FEHB coverage, and then tries to use the SEP later is in for a difficult conversation at the Social Security office. Retiree FEHB is not current-employment EGHP coverage. It does not extend the SEP. The retiree who delays Part B enrollment beyond the eight-month window under retiree FEHB coverage is treated as having delayed Part B without a valid SEP, and the late-enrollment penalty applies.
This is one of the more painful surprises in federal retirement. The retiree assumed — reasonably — that because they had FEHB coverage, the SEP would extend. It does not. The clock starts at retirement, not at the end of FEHB coverage.
How to use the SEP at retirement.
For a federal employee who is 65 or older and retiring, the cleanest sequence is:
- Confirm your retirement date with your agency’s HR office. The SEP starts the month after your active employment ends.
- Apply for Part B through Social Security using form CMS-40B (Application for Enrollment in Medicare Part B). The form is short. The supporting documentation is not.
- Provide proof of EGHP coverage based on current employment — usually a letter from the employer or the employer’s benefits office, on letterhead, confirming the dates of coverage and the basis (active employment).
- Choose a Part B effective date that aligns with the retirement date, generally the first of the month following retirement.
- Coordinate with FEHB to ensure no gap in coverage and no duplicate coverage during the transition month.
Social Security processes SEP-based Part B applications manually. The processing time can take several weeks. Most retirees should apply two to three months before their intended Part B effective date to avoid gaps in coverage.
What happens if you miss the SEP.
A retiree who misses the eight-month SEP does not lose the ability to enroll in Part B entirely. They can enroll during the General Enrollment Period (GEP), which runs January 1 through March 31 each year, with coverage effective the month after enrollment. The catch is the late-enrollment penalty: 10% of the standard Part B premium for every full 12 months of delayed enrollment, applied for life.
On 2026 numbers, the standard Part B premium is $202.90. A retiree who delays Part B for 24 months without a valid SEP pays a 20% permanent surcharge, or about $40 per month above the standard premium. Over a normal retirement, that surcharge easily reaches five figures. The penalty is mandatory. It does not expire. It does not reset. It is the most expensive line item in delayed Part B enrollment.
The documentation retirees most often get wrong.
The most common SEP-application error is missing or weak documentation. Social Security requires proof of EGHP coverage based on current employment for every month between the retiree’s 65th birthday (or IEP end) and the retirement date. The proof has to be specific about dates and the basis of coverage.
A letter that says “the retiree was covered under our group health plan from 2018 to 2024” is not enough if the retiree turned 65 in 2020. The relevant window for SEP eligibility is the months after the IEP, not the entire employment history. The letter has to cover the right months for the right reason.
For more on the larger Part B decision and the penalty that the SEP protects against, read what Medicare Part B is, and whether it is mandatory.