Paul Capece, a co-founder of PACS Architecture, explains, “A lot of people think they are going to buy the apartment next door and knock a hole in the wall. This isn’t how it works. If you combine two units and now have a 2,000-square-foot apartment, the scale of your apartment has changed, so you’ll need to renovate. For example, if you suddenly have 2,000 square feet, the small galley kitchen in your existing unit will now look undersized.” But Capece emphasizes that updating your kitchen is only the beginning. When combining apartments, owners must think carefully about the overall layout, circulation, windows, structural conditions, plumbing, electrical systems, and the way the finished home will feel as a single coherent residence.

Given the scope of most apartment-combination projects, Capece recommends consulting with an architecture firm early in the process, ideally before making a bid on the adjacent unit. Jorge Fontan of Fontan Architecture agrees, noting that it is not unusual for buyers to call an architect before they close. Using available documents, an architect can help determine whether a combination is feasible, sketch out a preliminary plan, flag likely complications, and estimate the potential cost of the work. PACS, like most architecture firms, charges a modest fee for this kind of preliminary review, but Capece says it is money well spent. “Once people consult with an architect, they understand the implications of combining two apartments and generally make smarter design decisions that also increase the resale value of the apartment.”

 

The question of financing a combination is often the first to consider. If you already own your apartment outright, the process may be easier, since your existing home equity can help you obtain financing to purchase the adjacent unit. Some banks may offer a cash-out refinance to purchase the second unit, while others may issue a single combination loan.

If you do not already own your unit outright, you may still be able to refinance or otherwise finance the purchase of the adjacent apartment, but you will need to qualify for a larger mortgage. You should also account for the cost of the renovation itself, which can be substantial and may require a separate construction loan or additional liquidity.

If you are purchasing two units at the same time, the financing may be more complicated. Lenders often scrutinize apartment combinations more closely than standard single-unit purchases, and buyers may face additional costs, including separate appraisals, inspections, and legal review for each apartment.

If you are trying to buy an adjacent unit in a co-op building, do your research before submitting an offer. Not all co-ops permit apartment combinations, and those that do may have detailed rules governing the process. Some buildings limit the number of renovations that can take place at once, while others restrict construction hours, require extensive board review, or impose alteration fees and deposits. Because renovation plans will need to be approved by the co-op board, it helps to understand the building’s culture and rules before moving forward.

Condos present a different set of issues. In a condo, each apartment is typically its own tax lot, and depending on the building, legal structure, and scope of work, the owner may or may not need to combine tax lots. Condo boards also usually require alteration agreements, insurance documentation, and professional plans before work can begin.

 

Finally, combining apartments is not just a private matter between an owner and a board. It is also subject to city rules. The New York City Department of Buildings has guidance for apartment combinations, and most legal combinations require filings, architectural plans, and inspections. DOB guidance generally requires the second kitchen to be eliminated and the plumbing connections capped unless approved plans show an alternate legal use for those connections, such as a washer/dryer, bar sink, or new bathroom. If your building is an individual landmark or located in a historic district, the Landmarks Preservation Commission may also need to review certain work, especially exterior changes, window work, or alterations that require DOB permits.

A basic physical connection between two units may sound simple, but a legal, well-designed apartment combination is much more than a doorway between apartments. The work usually involves architectural planning, board approvals, DOB filings, inspections, demolition, construction, and a redesign that makes the combined space feel intentional rather than stitched together.

Horizontal combinations are what most people imagine when they think of joining two adjacent apartments. Vertical combinations are also possible and can create dynamic duplex layouts, but they are often more complicated. Adding a staircase usually requires structural work, careful coordination with building systems, and additional approvals. Many buildings also have “wet over wet” rules that require bathrooms and kitchens to remain stacked above similar spaces below, which can limit flexibility when reworking a duplex plan.

With any combination, owners should expect to invest significant time and money creating a seamless connection between the units. Flooring, ceilings, lighting, millwork, doors, moldings, and mechanical systems often need to be coordinated throughout the new home. In older buildings, electrical upgrades can be especially expensive, since replacing older wiring may require opening walls and repairing finishes throughout the apartment. Before removing any wall, an architect or engineer must also determine whether the wall is structural and whether plumbing, electrical, gas, or other utilities run through it.

Windows can also become a major cost. In New York, replacements often run several thousand dollars per opening, and the cost can climb quickly in co-ops, condos, prewar buildings, or landmarked properties where approved window types and installation rules are stricter. Even a modest window scope can add tens of thousands of dollars to a project.

Capece explains that New York City owners should now treat $400 per square foot as a starting point rather than a ceiling for serious renovation work. Depending on the building, finishes, structural work, mechanical upgrades, and level of customization, apartment-combination projects can easily run $400-$600 per square foot or more, with high-end Manhattan or prewar combinations rising materially higher. In other words, even when two smaller apartments are being combined, the renovation budget can quickly move from tens of thousands of dollars into the hundreds of thousands.

When the project is finished, you may feel as if you are living in one apartment, but your building may not treat it that way financially. In a co-op, buying the adjacent apartment means buying additional shares, and additional shares typically mean higher monthly maintenance. In a condo, two combined units may still carry separate common charges and real estate tax bills unless a legal tax-lot combination is completed and accepted.

Boards may also charge alteration fees, review fees, move-in and move-out fees, security deposits, or fees for the building’s architect or engineer to review your plans. These costs vary widely from building to building, but they should be included in the early budget.

If you combine two apartments, your property taxes and carrying costs will likely increase. Because the federal SALT deduction is capped and subject to income limitations, higher real estate taxes can still have a meaningful impact on the true cost of owning a larger combined apartment.

There may also be tax questions if part of the new space will be used for work. The IRS simplified home-office deduction allows qualifying taxpayers to deduct $5 per square foot of home used for business, up to 300 square feet. However, this deduction generally applies only to qualifying business use and not to ordinary W-2 employees working from home. A percentage of certain related expenses may also be deductible for eligible taxpayers. Because the rules are specific and can vary depending on how the space is used, buyers should consult a tax professional before relying on any home-office deduction.

Finally, if you plan to sell in the future, you need to think carefully about resale value. Unless you plan to buy, renovate, and hold the property for many years, it may be difficult to recover the full cost of the combination. Even over a longer hold period, the resale value of the combined apartment will depend heavily on how well the project was designed.

Capece and his PACS Architecture colleague Jeff Massey say resale value ultimately reflects the quality of the design. Massey explains, “There is a definite premium for larger and well-designed apartments in New York City. But again, if you just knock a hole in the wall, people will immediately see that the apartments are a hodgepodge. You really want the units to look like one cohesive apartment.”

Capece agrees. “People are willing to pay a premium for apartments that are large enough for a family. Many people are coming from the suburbs or cities with more space, and they are accustomed to living with a certain scale. If they can find a well-designed apartment that offers that scale, they will usually be willing to pay more.”

The takeaway is simple: A successful combination should not feel like two apartments joined together. It should feel like one larger, better-planned home. When the architecture, layout, approvals, and budget are handled properly, combining apartments can create rare scale and long-term value in a city where both are hard to find.