Most apartment owners planning a renovation in New York City fall into one of two camps. The first group assumes their project is straightforward and that whatever permits are needed will get handled automatically. The second group has heard the terms ALT-1 and ALT-2 but isn’t sure which one applies to them, and is operating on guesswork. Both positions lead to the same outcome: surprises during the approval process that delay the project, increase costs, or create compliance problems that need to be resolved before construction can move forward.

Choosing the correct filing type at the outset isn’t a technicality. It determines how the project gets reviewed by the NYC Department of Buildings, what documentation is required, how long the approval process takes, and in some cases whether the project can be completed as designed at all. An architect who files the wrong type isn’t cutting corners; they’re filing inaccurately, which the DOB will catch. The project then needs to be re-filed correctly, the clock resets, and weeks or months of approval time are lost.

Understanding what these filing types mean, and more importantly what triggers each one, is one of the most practical things an owner can do before a renovation begins.

What Alteration Types Are and Why They Exist

The New York City Department of Buildings classifies construction work into categories based on scope, complexity, and the nature of the change being made to a building. These categories exist because not all work poses the same level of risk to occupants, neighboring residents, or the building’s structural integrity. The DOB uses filing types to direct the right level of scrutiny to the right kind of project.

One point of terminology first, because it trips people up. For years these filings were called ALT-1 and ALT-2 (with a minor ALT-3), and almost everyone — owners, architects, contractors — still says it that way. But in 2021 the DOB moved from the old BIS system to the DOB NOW: Build platform and renamed them. What used to be an ALT-1 is now filed as an Alteration-CO. What used to be an ALT-2, along with the old ALT-3, is now simply an Alteration. The classification logic behind the split hasn’t changed, but the names on the actual filing have. This guide uses the current names and notes the old ones alongside them, because you’ll hear both.

The old ALT-3 no longer exists as a separate filing type; DOB NOW folded that category into the general Alteration. The practical distinction between the old ALT-2 and ALT-3 was scope, not certification: ALT-2 covered work touching multiple trades or disciplines, while ALT-3 covered simpler filings limited to a single work type — a plumbing change on its own, say, with nothing else involved. That’s why ALT-3 was described as “minor”: it was narrower in scope, not procedurally different. It’s worth clearing up a related confusion here too: ALT-3 was often described as “self-certified,” but self-certification was never a property of that type. Professional Certification, where a licensed PE or RA certifies that the plans meet code and the filing skips plan examination, is a separate mechanism that can apply to different filings. It isn’t what “ALT-3” meant. For any renovation with real scope — relocated walls, new kitchens or bathrooms, new mechanical systems — the relevant question is almost always Alteration-CO or Alteration.

Alteration-CO (the Old ALT-1): When Major Changes Are on the Table

An Alteration-CO filing is required when the proposed work results in a change to the building’s use, occupancy classification, or egress. These are the three triggers, and each of them carries weight.

A change in use means the apartment’s legal function is being altered in a way the building’s existing Certificate of Occupancy doesn’t reflect. The most common example in residential renovation is apartment combination: merging two separate units into a single larger residence. The existing CO describes two apartments; the project creates one. That change in legal occupancy must be reflected in a new CO, and that requires an Alteration-CO filing.

A change in occupancy classification under the New York City Building Code involves moving a space from one legal occupancy group to another. For most residential apartment renovations, this doesn’t come up in an obvious way, but there are edge cases. Converting a portion of a residential apartment for professional use, adding a home office in a building where that use isn’t already contemplated in the CO, or changing a space’s designation in ways that alter the building’s classified occupancy can all trigger this requirement.

A change in egress is perhaps the most consequential trigger from a code standpoint. Egress refers to the means by which occupants can exit the building in an emergency: doors, corridors, stairs, and the paths connecting them. If a renovation alters the configuration of a corridor serving multiple units, relocates or modifies a doorway that affects the path of travel to an exit, or changes the egress layout in a way that modifies what the approved plans show, an Alteration-CO is required. In apartment renovations, this most often comes up when front entry doors are relocated or when work is done near building corridors.

The defining feature of an Alteration-CO is that it produces a new Certificate of Occupancy upon completion. The existing CO, which documents the legal use and configuration of the space, is superseded. The new CO reflects the completed work and becomes the legal record of what the space is. This is not a formality. The CO is a legal document that affects the apartment’s sale, financing, insurance, and occupancy. A project that requires a new CO but was filed as a plain Alteration has a fundamental compliance problem that will surface eventually, and the circumstances under which it surfaces — a sale, a refinancing, an insurance claim — are rarely convenient.

Alteration-CO filings are inherently more complex. They require a more comprehensive drawing set, additional documentation, and a more intensive DOB review process. The approval timeline is longer, the interaction with the DOB’s plan examination team is more involved, and the final sign-off requires the issuance of the new CO before the space can be legally occupied. For projects that genuinely trigger it, this complexity is unavoidable and shouldn’t be viewed as a problem — it’s the correct process for the scope of work being performed.

Alteration (the Old ALT-2): The Standard for Most Apartment Renovations

The Alteration filing is the workhorse of Manhattan apartment renovation. It applies to alterations that involve multiple work types, meaning they touch more than one trade or discipline, but don’t change the building’s use, occupancy classification, or egress. If the apartment starts as a two-bedroom residential unit and ends as a two-bedroom residential unit, and no one altered the path to the exit stair, the filing is almost certainly an Alteration (the old ALT-2).

The breadth of work that falls within it is substantial. A full gut renovation that relocates the kitchen, reconfigures the bathrooms, removes non-load-bearing interior partitions, installs new plumbing and electrical systems, and replaces all finishes throughout the apartment is typically filed as an Alteration — as long as none of those changes affect use, occupancy, or egress. The fact that the work is extensive and involves multiple trades doesn’t move it into Alteration-CO territory on its own.

Interior layout changes are among the most common scope items. Removing a non-load-bearing wall between a kitchen and dining area, opening up a corridor to create a more open floor plan, or reconfiguring bedroom arrangements within the existing apartment footprint all typically fall within an Alteration. The work requires architectural drawings, structural confirmation where walls are involved, permit issuance, DOB inspections, and a letter of completion at the end, but it doesn’t produce a new CO.

Kitchen and bathroom renovations almost always fall here, even when they involve relocating plumbing, changing fixture counts, or upgrading electrical service within the unit. These changes affect the apartment’s configuration but not its legal use or occupancy classification.

Mechanical system replacements and upgrades — new HVAC systems, updated electrical panels, replacement plumbing within the unit — are Alteration scope items. So is work involving structural elements, including removing a load-bearing wall with proper engineering and temporary and permanent support, as long as the structural work doesn’t affect the building’s egress configuration or alter the legal occupancy.

It’s worth being direct about what an Alteration is not: it is not a lesser permit or a shortcut. It requires a complete set of permit drawings prepared and stamped by a licensed architect, review by the DOB, issuance of a permit before work begins, DOB inspections during construction, and a formal letter of completion at the end. Owners who think “Alteration” means “easy permit” are setting themselves up for an inspection process they didn’t anticipate.

Alteration-CO vs Alteration in Practical Terms

The difference between the two isn’t primarily about how much work is being done — it’s about what the work does to the building’s legal record. That distinction has real consequences that play out throughout the project.

From a timeline standpoint, Alteration-CO filings consistently take longer to approve. The DOB’s plan examination process for them is more intensive, the drawing requirements are more comprehensive, and the interaction between the applicant’s architect and the DOB’s examiners involves more cycles of review and response. Where a clean Alteration submission might receive plan approval in four to eight weeks, an Alteration-CO for a project of similar physical scope can take eight to sixteen weeks or longer, particularly if the filing involves occupancy classification questions or egress modifications that require detailed code analysis.

There is one way to shorten that timeline meaningfully on the Alteration side: Professional Certification, where a PE or RA certifies that the plans comply with code and the filing moves straight to permit issuance instead of waiting in the plan examination queue. It’s a real option for many Alteration filings, though not a universal one — certain scopes are excluded, including demolition work in specific Anti-Harassment Districts — and a professionally certified filing is still subject to DOB audit after the fact. It’s less useful on the Alteration-CO side, because occupancy and egress questions are exactly the kind of judgment call plan examination exists to make, so many Alteration-CO filings go through full review regardless. Ask the architect early whether the project qualifies.

The documentation requirements diverge in meaningful ways. An Alteration requires architectural drawings covering the full scope of work: floor plans, reflected ceiling plans, partition schedules, plumbing diagrams, electrical layouts, and specifications. An Alteration-CO requires all of that plus a more complete code analysis, occupancy calculations, egress diagrams, and in many cases additional documentation supporting the change in use or occupancy. Structural calculations, fire-resistance ratings, and accessibility compliance documentation are more commonly required.

The cost implications follow from the documentation and timeline differences. An Alteration-CO requires more from the architect, more from any engineers involved, more from the expediter, and more time before the permit is issued and construction can begin. For owners operating on a fixed budget or a time-sensitive schedule, the distinction matters financially.

At the end of the project, the sign-off requirements differ. An Alteration closes out with a letter of completion from the DOB, issued after all required inspections have been passed. An Alteration-CO closes out with a new Certificate of Occupancy, which requires a final inspection that verifies the completed work against the approved drawings and confirms that the space complies with the approved occupancy and egress configuration. Missing or open inspection items that might be resolvable informally at the end of an Alteration project become more consequential when the project is waiting on a new CO.

Both filing types interact with co-op and condo board approval processes, but Alteration-CO filings tend to draw more board scrutiny. Because an Alteration-CO changes the building’s own Certificate of Occupancy, the board isn’t just reviewing your renovation — it’s reviewing a change to the building’s legal record, and that gets read differently in the board room. Budget board review as time layered on top of the DOB’s eight-to-sixteen-week window, not time that runs inside it; the two processes only partly overlap, and a board that wants its own engineer to weigh in on the CO implications can easily add a month or more before the alteration agreement is even signed. There’s also a newer step that applies to both filing types: since February 2026, when a co-op or condo unit owner files in DOB NOW, the board is automatically added as a stakeholder on the job and a board representative has to attest that they authorized the filing. Industry sources estimate this adds a few weeks to the permit timeline, so it belongs in the schedule from the start.

How to Determine Which Filing Applies to Your Project

The answer to which filing type applies is not something an owner should determine independently. This is one of the first substantive questions an architect addresses when they evaluate a project, and it requires professional judgment applied to the specific facts of the renovation.

That said, understanding the logic helps owners have more informed conversations with their architects and catch potential issues before they become expensive surprises.

The starting point is always the existing Certificate of Occupancy. The CO documents the building’s legal use, the classification of each space within it, the number of legal units, and the approved egress configuration. Any proposed work needs to be evaluated against what the CO currently reflects. If the proposed work changes anything the CO describes, an Alteration-CO is likely required.

The egress question requires careful analysis. It’s not always obvious whether a proposed layout change affects egress, and the answer depends on the full path from the apartment to the building exit, not just the interior of the unit. An architect who knows the building’s egress layout and understands how the DOB interprets egress requirements under the NYC Building Code can answer this question reliably. One who doesn’t may not catch an egress issue until the DOB’s plan examiner flags it mid-review.

Structural work is often mistakenly associated with the Alteration-CO, but structural modifications don’t automatically trigger it. Removing a load-bearing wall within an apartment requires engineering analysis, a structural engineer’s drawings, and careful coordination, but if it doesn’t change use, occupancy, or egress, it can typically be filed as an Alteration. The structural work is complex; the filing type is determined by the CO implications, not the complexity of the work itself.

Combination projects, where two adjacent apartments are being merged into one, always require an Alteration-CO. There’s no ambiguity here. Two legal units becoming one legal unit is a change in occupancy that requires a new CO, full stop. Owners who are told otherwise should ask for a written explanation of the code analysis supporting that position.

The Filing Decision Sets the Foundation for Everything Else

Every phase of a Manhattan apartment renovation flows from the decisions made at the beginning: scope, budget, design, and filing strategy. Of these, filing strategy is the one most owners pay the least attention to, and the one that most reliably determines whether the project moves through approvals smoothly or gets stuck.

Getting the filing type right from the start means the DOB review process addresses the correct scope of work, the drawing requirements are clear, the approval timeline can be estimated accurately, and the project’s legal records are handled properly from the beginning. It means the board submission reflects what will actually be filed, so there are no discrepancies between what the board approved and what the DOB is reviewing. And it means the project closes out correctly, whether that’s a letter of completion for an Alteration or a new Certificate of Occupancy for an Alteration-CO, with a clean legal record that will hold up through any future sale, financing, or occupancy question.

Which filing do you need for your NYC renovation? The answer starts with a conversation with a licensed architect who knows the building code, knows the DOB filing process, and is willing to give you a direct answer based on the actual scope of your project. That conversation is worth having before anything else gets decided.