Combining Two Apartments in NYC: What It Takes

An apartment combination joins two units into one home. In New York it is one of the few ways to get more space without moving. Buildings do not let you add floor area, so when the apartment next door or the one above comes up for sale, owners buy it and combine.

Combinations are more complex than a renovation of a single unit. Two sets of systems have to become one. Two legal units have to become one on paper. The building and the city both have to approve it. This article explains what is involved, in the order it usually happens.

Grandeur Hills Group handles combinations as part of our apartment renovation work in Manhattan and Brooklyn. Everything below is based on how those projects run.

The Two Types of Combination

Side by side. The two apartments share a wall on the same floor. You open the wall between them and re-plan the result as one apartment. This is the more common type and the simpler one, though the shared wall is often structural.

Up and down. One apartment is directly above the other. You cut an opening through the floor slab and build an interior stair. This creates a duplex. It involves structural work on the slab, fire-safety rules for the stair, and more engineering than a side-by-side combination. It also has to be designed so that each floor still works as a floor, not as two apartments joined by a staircase.

Who Has to Approve It

The building

In a co-op, the board approves the alteration, and the corporation typically has to merge the shares and proprietary leases of the two units into one. In a condo, the board approves the alteration, and the two units are usually merged in the condominium documents so that one owner holds one unit with one common-interest percentage. Both cases involve the building's attorney, not just its managing agent. Ask early what the building requires, because some buildings set conditions on combinations that affect the design — for example, requiring that the second kitchen be removed.

The city

A combination is filed with the Department of Buildings by a registered architect or engineer. Because it changes the number of dwelling units, it may require an amended certificate of occupancy for the building. Your architect determines the filing type. Plumbing, electrical, and structural work are permitted and inspected in the normal way. If the building is landmarked and any exterior element changes, the Landmarks Preservation Commission is involved as well.

We cover the general approval process — alteration agreements, board packages, DOB filings — in our guide to renovating a co-op or condo in NYC. A combination goes through all of that, plus the legal merger of the units.

What Changes in the Construction

• The wall between the units. In many prewar buildings the demising wall is masonry and may carry load. An engineer confirms what can be opened and how. In some cases the opening is sized and framed with steel; in others the wall has to stay and the plan works around it.
• The second kitchen. Most buildings require the second kitchen to be removed so the result is one dwelling unit. That means capping gas and plumbing lines and removing the wet room properly, not just taking out the cabinets. Some owners convert the former kitchen into a laundry, a bar, or a pantry, which keeps some plumbing and is subject to the building's wet-over-dry rules.
• The bathrooms. The combined apartment usually has more bathrooms than it needs, in the wrong places. Removing or relocating them is governed by the plumbing stacks and by what sits below each one.
• Electrical. Two units means two panels and often two meters. Depending on the building and the utility, the service is consolidated into one panel, or the second meter is kept and the two panels are tied together. This requires coordination with the building and the utility company and is one of the items that takes longer than owners expect.
• Heating and cooling. Two apartments were conditioned separately. The combined plan needs one system that serves the new layout, or the existing units re-zoned to match it.
• Entry doors. Each unit had its own front door. The combined home typically keeps one as the main entry and, if the building allows, the other as a service entrance. Fire code and the building's rules decide whether a second door can be removed or must stay.
• The stair (vertical combinations). Cutting the slab, framing the opening, and building a stair that meets code. This is structural work and requires the engineer, the DOB, and usually the building's engineer to sign off.

The Design Problem

The most common mistake in a combination is opening the wall and keeping everything else. The result is two apartments with a doorway between them: two living rooms, two entries, bedrooms on opposite sides of the home, and a long corridor.

A good combination is re-planned from scratch as a single apartment. That means one entry and one arrival sequence, one kitchen in the right place, bedrooms grouped together, and public rooms grouped together. It means using the second unit's former living room as something else if the plan calls for it. It usually means moving more walls than owners first expect, which is why the structural and plumbing investigation has to happen before the design, not after.

Two practical things to check early. First, floor levels: the two units may not sit at exactly the same height, and the transition has to be resolved, not hidden under a threshold. Second, ceiling heights and window orientation: the two units may differ, and the plan should put the rooms where the light and the height are.

Order of Operations

• Close on the second unit first. Do not start design until you own both apartments. Purchases fall through, and a design drawn for a unit you do not own is wasted work.
• Confirm what the building requires before designing. Rules on second kitchens, second entries, and required completion periods are design constraints.
• Investigate the structure and plumbing. The shared wall, the slab, the stacks, and the existing conditions behind both units' walls.
• Design the combined apartment as one plan. Then prepare the board package and DOB filing from that plan.
• Approvals, then construction. The approval phase is longer than for a single-unit renovation because of the legal merger and the possible certificate-of-occupancy change. The construction phases are the same as any full renovation; we describe them in the NYC gut renovation timeline.
• Close out the paperwork. DOB sign-off, the building's final approval, and the merged unit documents. A combination that is not properly documented can be a problem when you sell.

Problems That Come Up Most Often

• The neighbor does not sell. Owners sometimes renovate their existing unit while waiting for the second one. If the design assumes a combination that never happens, that renovation has to be undone later. Renovate the unit you own as a complete home on its own.
• The wall cannot open where the plan needs it. This is discovered during the structural investigation, or during demolition if that investigation was skipped. It is the reason to investigate first.
• Utility and metering delays. Consolidating electrical service involves parties outside the project. Start that process early.
• Deadlines in the alteration agreement. Some buildings set a completion period. A combination is a large scope; make sure the agreed period fits the realistic schedule before signing.
• Resale documentation. Buyers, lenders, and title companies look for a properly merged unit and a matching certificate of occupancy. Finish the legal side, not just the construction.

How We Handle Combinations

We run combinations as design-build projects. Our architects investigate both units, design the combined plan, and prepare the board package and DOB filing. The same team then builds it. This matters more for a combination than for most projects, because the structural, plumbing, electrical, and legal questions all affect each other, and one team can resolve them together. We have explained the design-build model in The Design-Build Advantage.

If you are considering buying the apartment next door or above, request a consultation before you close. We will tell you what the building is likely to require, what the structure will allow, and whether the two units can be planned as one good home.