Key takeaways
- A Maryland S corporation files Form 510 if it did not elect entity-level tax, and Form 511 if it did.
- For a calendar-year entity the tax year 2026 return is due April 15, 2027, a month after the federal Form 1120-S.
- The 7-month extension on Form 510/511E does not extend the time to pay.
- Collect the election history and nonresident shareholder facts in November, before the K-1s are drafted.
Which Maryland return an S corporation files
The regulation on filing returns, COMAR 03.04.07.03, sends a pass-through entity to Form 510 as an information return. An entity that elected entity-level tax files an income tax return on Form 511 instead. The Comptroller calls the estimate form Form 510/511D and the extension Form 510/511E. So the first question for every S corporation client is whether Maryland treated them as electing or nonelecting for the year.
The regulation also requires a Schedule K-1 for each nonresident member. The Comptroller's own K-1 is the 510/511 Schedule K-1, and its tax alert says net capital gain is reported separately to each member on it. A federal K-1 does not finish the state work.
On electronic filing, the 2025 Comptroller booklet requires Form 510 to be e-filed if the entity generated a business tax credit on Form 500CR to pass to members. Form 510/511E can go through MeF, online or telefile. I found no general e-file mandate for entities without credits.
Due dates for tax year 2025 and tax year 2026, side by side
Federal first. Form 1120-S is due the 15th day of the 3rd month after year end: March 16, 2026 for the tax year 2025 calendar-year return, and March 15, 2027 for tax year 2026. A federal Form 7004 gives 6 months and does not give you Maryland's date.
Maryland is later. Under COMAR 03.04.07.03, an S corporation return is due the 15th day of the 4th month after the close of the tax year, with an automatic extension of up to 7 months. Request it on Form 510/511E by the original due date.
Tax year 2025, calendar year: Maryland due April 15, 2026, extended to November 15, 2026. That date falls on a Sunday, so confirm the Comptroller's weekend rule before you tell a client the last day. These are the returns still open this month.
Tax year 2026, calendar year: Maryland due April 15, 2027, extended to November 15, 2027.
Fiscal-year entity: the rule counts from the close of the tax year, so a June 30 year-end puts the Maryland return at October 15 and the extended date at May 15, while the federal date is September 15. Count both from year end; do not assume April.
The regulation says an extension to file may not extend the time for payment. If the entity pays tax for nonresident shareholders or owes entity-level tax, the money is due on the original date however long the return waits.
Estimated payments and the year-end catch-up
COMAR sets S corporation estimates due on or before the 15th day of the 4th, 6th, 9th and 12th month of the tax year: April 15, June 15, September 15 and December 15 for a calendar-year entity. Estimates are required when the tax is reasonably expected to exceed $1,000 for the year.
Three of the four tax year 2026 dates have passed. Pull the client's Maryland payments to date, compare them with a rough estimate of Maryland-source income, and set the December 15 payment before the return shows a balance.
The pass-through entity election: what applies to which year
By default Maryland taxes the nonresident members' shares at the entity level. If the entity elects, it pays tax on the shares attributable to Maryland, files Form 511, and members take a credit rather than paying that income again.
The general rule, in the Comptroller's Technical Bulletin No. 6 and again in Tax Alert TA-13-APR-2026PTE, is that the election is made with the first filing or payment of the tax year and is irrevocable for that year. Tax year 2026 has a one-time exception: the alert says the Comptroller will ignore any election or nonelection made with the first-quarter estimate and will honor the one made with the next filing or payment after April 15, 2026, usually the second-quarter estimate.
For a tax year 2026 file, ask what the client paid on June 15, 2026, and which election Form 510/511D reflected. For tax year 2025, look at the first payment of that year. Do not copy the 2026 exception onto a 2025 file.
The alert says the 2026 budget act postponed changes to how a pass-through entity's taxable income is calculated to tax year 2027, with guidance to be issued separately. For tax year 2026, electing entities are taxed on shares attributable to Maryland, as in tax year 2025. I found no published detail on 2027, so do not plan around it.
For an electing entity the alert gives the rate as the lowest county income tax rate plus the highest marginal individual rate on individual members' shares, plus the corporate rate on entity members' shares. Take the percentage from the current Form 510/511D instructions.
The 2% tax on net capital gain applies to individuals with federal adjusted gross income over $350,000. The entity does not pay it for members, and an overpayment is refunded to the entity, not passed through. A shareholder with a large gain needs a personal estimate that covers it.
Nonresident shareholders, the composite return and a worked example
For a nonelecting entity, the nonresident shareholders drive most of the Maryland work. Each needs a Maryland K-1 showing the Maryland share, and the entity pays tax on that share. The K-1 also supports a credit for taxes paid to Maryland on the shareholder's home-state return.
COMAR 03.04.02.04 lets an S corporation file a composite return for nonresident members who are individuals and elect in writing. Technical Bulletin No. 6 says it is on Form 510C and only a nonelecting entity can file one. It is due with the information return, and the president or another authorized officer signs it.
Worked example, using the tax year 2023 rates in Technical Bulletin No. 6 only to show the arithmetic: the special nonresident tax was 2.25% and the top individual rate 5.75%, together 8%. A nonresident individual owning 40% of an S corporation with $250,000 of Maryland-source income has a $100,000 Maryland share, and the entity-level tax at 8% is $8,000. A nonresident entity shareholder was taxed at the 8.25% corporate rate. For 2026, take the rates from the 2026 Form 510/511D instructions and repeat the steps: member share, rate by member type, total.
Mid-year residency changes are the weak spot. Neither COMAR nor Technical Bulletin No. 6 says how to split a member who moves into or out of Maryland during the year, so get the move date in writing and follow the current Form 510 instructions for the K-1.
Documents to collect from the client
Request: "Please upload the Maryland Form 510/511D you filed or paid with in April and June 2026, and tell us whether the company elected to pay tax for all members." Without that you cannot tell which return you are preparing.
Request: "Send a current shareholder list with each person's home address on December 31, 2026, ownership percentage, and the date of any move into or out of Maryland." Residency and any mid-year sale split resident from nonresident shares.
Request: "List every state where employees worked or jobs were performed, with revenue and payroll by state." That feeds Maryland apportionment and flags other states.
Also collect the prior-year Form 510 or Form 511, the Form 510/511E, payment confirmations for April 15, June 15, September 15 and December 15, signed composite election letters, any Form 500CR credit paperwork, and the federal draft with each Schedule K-1 (Form 1120-S). Usual can send this as one document request and show which items are still missing.
The federal penalty that sets the pace
The current Form 1120-S instructions give a penalty of $255 for each month or part of a month, up to 12 months, multiplied by the number of shareholders, when the return is late and no tax is due. The federal return and K-1s drive the Maryland return, so work to March 15, 2027, and treat April 15 as the state deadline only.