Combining two NYC apartments: Why a pre-purchase assessment is the key to success
- Buying an adjoining unit suits New Yorkers seeking larger and more customized spaces
- Gallery KBNY, a design-build firm, conducts pre-purchase site visits to determine a project's scope
This state-of-the-art kitchen was part of an overhaul of a previously combined two-unit condo in the iconic Chelsea Mercantile for a family of five who likes to cook. The massive island serves as the centerpiece of the 25-by-10-foot space.
Gallery KBNY
Rather than trade a beloved building or neighborhood for more space, a growing number of New Yorkers are looking sideways—or up. They're buying the apartment next door, or the one above (or below), to create a home that suits how they live today and their needs in the years ahead.
"One of the biggest trends we're seeing is that more people are choosing to combine apartments instead of moving," said Alex Ushyarov, director of business development at Gallery KBNY, a Manhattan-based full-service design-build firm.
Limited inventory of larger apartments, coupled with the high cost of purchasing a move-in-ready home, has made expanding into an adjacent unit an increasingly attractive alternative. At the same time, homeowners are prioritizing home offices, guest/flex rooms, and renovations tailored for longer-term living.
But while apartment combinations can unlock tremendous potential, the projects that succeed aren't necessarily the ones with the biggest budgets or the boldest designs. They're the ones where the most important decisions are made long before demolition begins—or the contract is ever signed.
That shift has changed the way homeowners seek professional guidance. "Increasingly, a significant number of clients want to understand feasibility, costs, and potential layouts before purchasing the neighboring apartment, especially as project sizes continue to increase," he said. "At Gallery KBNY, we perform these pre-purchase renovation assessments regularly so clients know exactly what they're getting into before making such a significant investment."
What to expect
Apartment combinations typically take around a year from start to finish. Ushyarov said to plan on five months for the planning, design, and approvals, followed by another five to six months for construction. As with any renovation, the exact timelines vary depending on the building, the scope of work, and the review process.
Costs also vary widely, but most full-apartment combinations start at $550 per square foot, while larger, more customized projects can exceed $700 per square foot. Factors such as structural work, high-end finishes, and the building's complexity all influence the final outlay.
One expectation is nearly universal: you should plan to live elsewhere during construction. Between demolition, structural work, utility shutdowns, and inspections, apartment combinations are rarely compatible with day-to-day living. Factoring temporary housing into your budget and timeline from the outset can make the renovation process far less stressful.
Before you buy
The most successful apartment combinations begin with a clear understanding of what's possible—and what isn't.
If you aren't sure, the first step is to check whether other apartments in the building have already been combined. Having precedent doesn't guarantee approval, but it can be a positive sign and help make the board process smoother.
Beyond that, the next step is to schedule a site visit with a reputable design-build firm or an architect to conduct a feasibility assessment. Two apartments that seem like a natural fit don't always combine as easily as they appear. The goal of this early investigation is to understand the project's true scope—including its budget, timeline, and any potential obstacles—before committing to the purchase.
"While it's impossible to eliminate every unknown in an existing building, thoughtful planning and thorough due diligence dramatically reduce risk and allow you to respond to the unexpected without compromising the overall design," Ushyarov said.
For example, in a current Upper West Side horizontal apartment combination, Gallery's team suspected an electrical conduit ran through the shared wall, but wasn't permitted to perform exploratory probes until after the adjoining apartment had been purchased. Rather than wait for demolition to dictate the design, the team developed two entry layouts in advance, ensuring the client's overall vision could move forward regardless of what they discovered. Exploratory work also revealed a slight difference in floor elevations between the two apartments—another condition the team had anticipated and accommodated without redesigning the project.
Navigating the approval process
Getting your board to sign off on the renovation package is an essential prerequisite. Whether you are combining apartments side by side or stacking one above the other, the board's primary concerns are ensuring the work is structurally sound, won't negatively affect neighboring apartments or building systems, and complies with the alteration agreement.
The quality of the initial submission makes all the difference—as does having an experienced team who can anticipate questions before they're asked. If you are adding central air conditioning, for example, the mechanical drawings should be included in the plans rather than waiting for the reviewing engineer to request them. Likewise, projects involving lowered ceilings should include those details upfront to avoid unnecessary review cycles.
It's important to ask how the building wants the finished apartment to be structured. Some co-ops prefer to leave combined apartments legally separate, allowing a future owner to divide them again. Others require the apartments to be permanently merged into a single legal dwelling. Those expectations vary by building and are typically established during the board approval process, Ushyarov said.
Even with a thorough submission, most projects require at least two rounds of review before receiving building approval, a process that typically takes six to eight weeks. Only then can plans be filed with the Department of Buildings (DOB)—and, if applicable, the Landmarks Preservation Commission—adding another six to eight weeks before permits are issued.
Note that because vertical combinations involve creating an opening in the floor slab for an internal staircase, these projects typically require closer scrutiny and a more extensive DOB permitting process.
In a recent Battery Park project where the clients had the rare opportunity to purchase the apartment directly above their existing townhouse, Gallery's team not only created a functional staircase, but one that became the focal point, bridging all three levels and redefining the home’s flow. To support the vertical combination, the Gallery team installed a steel beam on the second floor, seamlessly tied into the design with soffits and closets to preserve flow, while exposed in certain areas for an industrial touch. This behind-the-scenes reinforcement allowed the expanded structure to integrate appropriately without compromising aesthetics.
Because experienced design-build teams understand what reviewing engineers and building officials are likely to ask for, they can often address those questions before they become delays. Once permits are approved, that same attention to detail carries into the field.
Before construction is underway
Even after permits have been secured, the investigative work isn't over. Before demolition begins, experienced design-build teams continue verifying existing conditions to avoid unexpected discoveries that would force costly redesigns.
One of the first priorities is confirming exactly what's hidden behind the walls. Rather than relying solely on building drawings or the reviewing engineer, Gallery's team performs exploratory probes before finalizing the design whenever possible. That allows them to determine whether a wall is structural, identify hidden columns, plumbing risers, gas lines, electrical conduits, and other concealed conditions, and adjust the design before construction begins.
Plumbing is another area that requires careful planning. Because most Manhattan apartments are built with concrete floors and ceiling slabs, plumbing can't simply be rerouted wherever it's most convenient. "One of the very first steps is identifying the exact locations of all plumbing risers so we know what is and isn't feasible before the design progresses too far," Ushyarov said. Fixture locations must be coordinated with those of existing risers to maintain proper drainage while still achieving the design goals.
"Ultimately, the common theme is the need to conduct a thorough due diligence before a single plan is submitted to the building," Ushyarov said. "Spending a few weeks upfront investigating the existing conditions can save months of delays and many thousands of dollars once construction begins."
If a renovation or apartment combination is on your horizon, an early conversation can help clarify what's possible. Gallery KBNY welcomes inquiries from homeowners planning projects throughout New York City. Reach the team at [email protected] or 718-690-3840.
