Combining two apartments in New York City sounds deceptively simple: remove a wall, unify the layout, and double the space. In reality, it is one of the most complex residential alterations you can undertake. Done well, it creates something rare in the city—a seamless, expansive home that feels intentional rather than stitched together. Done poorly, it results in awkward circulation, compromised structure, and months (or years) of delays.

Having worked through a full apartment combination—like the Lafayette project—I can say that success depends less on aesthetics and more on navigating constraints: regulatory, structural, and infrastructural. Design is only one piece of the equation. The real work is coordination.

1. Start with the Building, Not the Plan

Before drawing a single line, you need to understand the building. In NYC, the building dictates what is possible far more than your design intent.

Key questions:

• Is the building a co-op or condo?

• What are the board’s alteration policies?

• Where are the structural walls and columns?

• How are plumbing stacks and risers distributed?

• What are the fire-rating and egress requirements?

Co-op boards, in particular, can be the gatekeepers. Many have strict rules about wet-over-dry conditions (no new bathrooms over living rooms), work hours, and even the scope of demolition. Some will outright reject combinations that affect structural elements or building systems.

This is where most projects are won or lost—before design even begins.

2. The Role of the NYC DOB (Department of Buildings)

Any apartment combination that involves removing walls, altering plumbing, or modifying mechanical systems requires filing with the NYC Department of Buildings (DOB).

Typically, this is an Alt-1 application (Alteration Type 1), because you are changing the legal occupancy configuration—two units becoming one.

This triggers:

• Updated Certificates of Occupancy (if applicable)

• Full plan examination

• Compliance with current building codes (not the code when the building was constructed)

This last point is critical. Combining apartments can force you to “upgrade” portions of the apartment to current code, including:

• Fire separation requirements

• Smoke and carbon monoxide detectors

• Egress widths and door clearances

You are not just merging two units—you are, in many ways, re-permitting them under today’s standards.

3. Structural Reality: The Wall Is Never “Just a Wall”

The biggest misconception clients have is that the wall between two apartments can simply be removed.

In NYC buildings—especially prewar—walls often:

• Carry structural loads

• Brace floor systems

• Contain critical utilities

If the wall is load-bearing, removal requires:

• Structural engineer analysis

• Design of a beam or lintel

• Temporary shoring during construction

• DOB approvals and inspections

Even when a wall is non-load-bearing, it may conceal risers, electrical conduits, or gas lines. Relocating these is often more complex (and expensive) than dealing with structure.

In one project, what appeared to be a simple opening revealed a vertical stack that served multiple floors. That single discovery reshaped the entire design.

4. MEP Systems: The Invisible Constraint

Mechanical, Electrical, and Plumbing (MEP) systems are the hidden architecture of any apartment—and the most difficult to reconfigure.

Plumbing:
Bathrooms and kitchens are typically tied to vertical stacks. Moving them even a few feet can trigger:

• Slab penetrations

• Coordination with units below

• Board approvals for wet-over-dry conditions

Most combinations work best when wet areas stay close to existing stacks. Ambitious relocations are possible, but they escalate complexity exponentially.

HVAC:
Older buildings may rely on through-wall units or steam radiators, while newer ones use centralized systems. Combining units can:

• Disrupt load calculations

• Require rebalancing or new zoning

• Trigger facade or penetrations issues if new units are added

Electrical:
Each apartment typically has its own panel and service. Combining units often involves:

• Consolidating panels

• Upgrading service capacity

• Coordinating with the utility (e.g., Con Edison)

Ignoring MEP early leads to compromised layouts later.

5. Designing for Seamlessness

The goal is not just to combine two apartments—it’s to erase the evidence that they were ever separate.

This requires discipline in:

• Circulation: Avoid long corridors that reveal the former boundary.

• Proportion: Align openings and ceiling heights where possible.

• Hierarchy: Establish a clear sequence of public to private spaces.

Often, the original entry doors become a design problem. Do you keep both? Eliminate one? Convert one into a service entrance or mudroom?

The best solutions treat the combination line as an opportunity rather than a limitation—placing key moments (a gallery, a library, a threshold) exactly where the seam once was.

6. Acoustic and Fire Separation

When you remove the dividing wall between units, you are also removing a layer of acoustic and fire separation.

NYC code and building requirements may require:

• Reintroducing fire-rated assemblies in new configurations

• Maintaining or upgrading floor/ceiling sound attenuation

• Ensuring penetrations are properly sealed

This is especially critical in co-ops, where neighbor impact is closely scrutinized.

7. The Approval Process: More Political Than Technical

Even with perfect drawings, approvals can be unpredictable.

You will likely need:

• Architect’s plans

• Engineer’s reports

• Contractor insurance and licensing

• Alteration agreements with the building

• Board approval (often including an interview)

Boards are not just reviewing technical compliance—they are assessing risk. Noise, duration, impact on neighbors, and the reputation of your contractor all matter.

A well-prepared submission package, with clear intent and minimal ambiguity, can shorten timelines significantly.

8. Construction: Where Theory Meets Reality

No matter how thorough the drawings, surprises are inevitable once walls are opened.

Common discoveries:

• Unmapped pipes or conduits

• Uneven floor levels between units

• Structural irregularities in older buildings

Contingency planning is essential—both in budget (typically 10–20%) and schedule.

In practice, the cleanest designs are those that can absorb small deviations without collapsing conceptually.

9. Cost: Why It’s More Than Double

Combining two apartments is not simply the cost of renovating one apartment times two.

Additional costs include:

• Structural interventions

• MEP reconfiguration

• DOB filing and expediting

• Extended construction timelines

• Higher design and coordination fees

In many cases, the cost per square foot is higher than a standard renovation because of the complexity involved.

10. Why It’s Worth It

Despite the challenges, combining apartments remains one of the most powerful ways to create value in NYC real estate.

You gain:

• Larger, more flexible living spaces

• Improved light and air (especially with corner combinations)

• A level of customization rarely possible in single units

More importantly, you create something that doesn’t really exist in the standard market: a home designed as a whole, rather than adapted from constraints.