Renovations in New York City are generally filed with the Department of Buildings as one of two job types, an Alteration or an Alteration-CO. The difference depends on the Certificate of Occupancy, often shortened to CO.
The CO is the city record that states how a building or space may legally be used. It can identify the approved occupancy classification, how many people a space may hold, and, in residential buildings, how many dwelling units it contains. Dwelling units are the separate legal apartments or residences in the building.
Some older New York City buildings do not have a modern CO. When that happens, the same question still applies, but the answer comes from the building's other legal records.
Work that changes the CO or legal occupancy record is filed as an Alteration-CO. Work that keeps that record in place is filed as an Alteration.
Older labels still come up in conversations about NYC permitting. ALT-1 generally corresponds to Alteration-CO. ALT-2 and ALT-3 generally correspond to Alteration. The current filing decision is simpler than the old names suggest. The question is whether the work changes the building's legal record.
What each type covers
An Alteration covers work that changes a building or space while keeping the existing Certificate of Occupancy in place. Interior layout changes, kitchen and bath renovations, new plumbing fixtures, electrical upgrades, storefront work, and many apartment or commercial fit-outs can fall here, even when the scope is large. A fit-out is work that prepares a space for a tenant or owner's use.
The filing is still an Alteration when the completed work matches the legal use, occupancy classification, egress, and dwelling-unit count shown by the applicable building records. Egress means the building's exits and the routes that lead to them.
An Alteration-CO covers work that changes the building's legal occupancy record. That includes a new use, a different occupancy classification, a change in the number of dwelling units, a change in the number of stories, or a major change to required egress. Occupancy classification is the building-code category that helps determine how a space may be used and how many people it may hold. DOB ties these triggers to Administrative Code 28-118.3.
An Alteration-CO ends with a new or amended Certificate of Occupancy, rather than a Letter of Completion.
The size of a renovation is not a reliable indicator of the job type. A full gut renovation may stay an Alteration if the building or space is legally the same when the work is finished. A smaller scope may require an Alteration-CO if it changes the legal use, occupancy classification, dwelling-unit count, story count, or required egress.
That is why a retail space becoming an eating and drinking establishment, the city's zoning term for a restaurant or bar, can be a bigger filing question than a large apartment renovation. Adding floor area needs the same judgment. A horizontal enlargement, meaning floor area added out to the side, that keeps the same use, occupancy, story count, and CO conditions can be treated differently from a vertical enlargement, meaning a story added on top. The direction of the addition matters, and so does what the Certificate of Occupancy must show once the work is complete.
Work that often stays as an Alteration
An apartment renovation usually files as an Alteration when it keeps the home in residential use, keeps the approved occupancy condition, and does not require the Certificate of Occupancy to change. A commercial tenant fit-out can also be an Alteration when the space stays within its approved legal use and occupancy.
The drawings may still be detailed, and the job may still carry several work types. Work types are the trade-by-trade categories a filing is divided into, such as general construction, plumbing, mechanical, sprinkler, or structural. The job type depends on whether the approved legal record changes.
Apartment combinations are a useful example because they show where the dwelling-unit rule has an exception. Adding dwelling units usually amends the Certificate of Occupancy and files as an Alteration-CO. Combining apartments can stay an Alteration when the project meets DOB's apartment-combination rules.
Under Local Law 77 of 1968 and DOB's Technical Policy and Procedure Notice 3/97, a qualifying apartment combination can file as an Alteration, with no new or amended CO. Among the conditions, the units have to sit on the same floor or on adjacent floors joined by an interior stair across no more than two stories, the result has to keep an equal or lower number of zoning rooms, and the building's bulk cannot increase. A zoning room is a habitable room as the city's zoning rules count it, and bulk is the building's overall size. A combination that meets the conditions closes with a Letter of Completion, even though the layout changes substantially. A combination that falls outside them can require an amended CO.
Work that often becomes Alteration-CO
A project tends to move into Alteration-CO when it changes the legal use of a building or space, changes the occupancy classification, adds dwelling units or otherwise changes the count in a way that amends the CO, changes the number of stories, or substantially changes required egress.
Egress is one of the places where the answer depends on the details. A localized door or corridor adjustment may stay inside an Alteration. A change that alters the building's required exit system can bring the project into Alteration-CO. The answer depends on how the building is approved to function after the work.
Alteration vs Alteration-CO
| Question | Alteration | Alteration-CO |
|---|---|---|
| Certificate of Occupancy | Keeps the existing CO in place | Requires a new or amended CO |
| Common work | Interior renovations, layout changes, plumbing or electrical upgrades, fit-outs, some apartment combinations | Use changes, occupancy changes, added stories, dwelling-unit changes that affect the CO, major egress changes |
| Review character | Usually more contained | Typically deeper, with occupancy and CO review |
| Typical closeout | Letter of Completion | New or amended Certificate of Occupancy |
| Timeline concern | Approval and inspections | Approval, inspections, occupancy review, and CO issuance |
| Main risk if misfiled | Corrections, objections, or amendments | Possible withdrawal and refiling if the job type is wrong |
How the filing moves through DOB
The architect or engineer of record, the licensed professional who prepares and submits the application, works through DOB NOW, the city's online platform for building filings. The application identifies the property, selects the job type, and adds the work types involved in the project. A single Alteration can include several, such as general construction, structural, plumbing, mechanical, sprinkler, standpipe, or fire alarm.
A filing moves through review in one of two ways. Under standard plan examination, a DOB examiner reviews the drawings and issues objections that the design team answers. Under professional certification, available for eligible jobs, the licensed architect or engineer certifies that the plans meet code, which can shorten initial approval. That route carries professional responsibility and can be audited by DOB.
For Alteration-CO filings, the Certificate of Occupancy is central to the review. The Schedule of Occupancy lists the approved use and occupancy information floor by floor. It has to line up with the drawings, required items, inspections, and closeout documents that support the new or amended CO. Professional certification can speed parts of the approval, but the occupancy work still has to be done.
How each type closes out
An Alteration generally closes with a Letter of Completion once the required inspections and sign-offs are done. That letter records that the permitted work is finished and that no new or amended Certificate of Occupancy was required.
An Alteration-CO closes with a new or amended Certificate of Occupancy. A Temporary Certificate of Occupancy, or TCO, can allow use of the space for a limited period while the last items are finished. The final CO is the permanent version.
On an Alteration-CO, the construction can be complete while the CO items are still open. For a project that changes use, adds units, changes occupancy, or changes required egress, the owner may still be waiting on those items after the trades have finished.
Why the filing type matters early
The job type is the first decision in a filing, and much of what follows depends on it. It sets the drawing set the design team prepares, the kind of review the application goes through, the documents the Department of Buildings requires, the inspections that have to pass, and the way the project closes out.
It also affects sequencing when one property carries several related filings. A Certificate of Occupancy applies to the whole building, and a property can have only one final CO. When two Alteration-CO jobs are open at once, the final CO is issued only after the last of them signs off.
The job type is set when the filing is created, and it cannot be changed afterward. When the architect or engineer of record sets up the application, the answers to a few opening questions determine whether the job is an Alteration or an Alteration-CO. If the wrong type was chosen, the correction is to withdraw the filing and submit a new one, which restarts review.
The choice comes down to two questions. What does the building's Certificate of Occupancy, or its legal record where no CO exists, allow today? What will the building legally be once the work is finished?
When the two answers match, the project is usually an Alteration. When the finished building differs from that record in use, occupancy classification, dwelling-unit count, story count, or required egress, the project is usually an Alteration-CO, and amending the CO is part of the schedule from the start.