Office-to-Residential Conversion: A Gensler Project Director’s Six-Step Checklist

In September 2022, I submitted what I thought was a fully coordinated drawing set for a 14-story office-to-residential conversion. Two weeks later, the contractor's RFI log came back with 34 items. Most went back to one gap: we had drawn ceiling heights without checking the existing beam depths across part of the building. We lost six weeks and reissued close to eighty sheets.

Since 2015 I've worked as a project director at Gensler, moving between architecture and interior design on adaptive reuse projects. I've personally made and documented seven significant mistakes on office-to-residential conversions. Add up the correction hours and I'd say the waste was somewhere north of $140,000—I'd have to check our spreadsheet for the exact number. This article is the checklist I now use to stop those mistakes from repeating.

If you own or manage an office building considering conversion, or you're a developer working to a fixed completion date, this list is for you. There are six checks, in the order that has worked for us. The order matters, and the one most teams skip is the last one.

1. Survey the actual building before you draw the units

Office buildings are deliberately repetitive. That repetition is what makes conversion feasible. But a building that works well as open office space does not necessarily repeat in ways that work for apartments. Window mullions, spandrel heights, floor-to-floor heights, and column sizes at the lower floors all become design drivers.

People told me early on not to trust the landlord's CAD file. I only believed them after my first conversion in 2017. We laid out an entire floor plan from the existing drawings, which were accurate for the structural grid but not for the glazing. Near the corners, the window module was about four inches different from what the drawings showed. In an open office, that meant nothing. In our unit plans, it meant the kitchen windows and bathroom vents were off by several inches on a lot of units. We caught it late, and the redraw took three weeks.

What we do now: commission a laser scan of the facade and spot-check floor-to-floor dimensions before organizing a single unit plan. It feels like an extra cost at the start. Compare that with the cost of a three-week redraw.

2. Run the natural-light and egress checks before you lock the unit mix

An office floor plate can be deep because offices can be mechanically ventilated and lit in ways that apartments cannot. Apartments need windows in habitable rooms. The first time you place an apartment plan onto a deep office floor plate, the space in the middle looks like a gift. It's not. It's a code problem.

Most U.S. jurisdictions that adopt the International Code Council's 2021 IBC require a habitable room to have glazing area of at least 8 percent of the room's floor area. Many also require openable area of about 4 percent for natural ventilation, unless mechanical ventilation is provided. Local amendments are common—verify the edition adopted where your project sits.

I'm not a code consultant, so I won't try to give you a jurisdiction-by-jurisdiction list. What I can tell you from a design and coordination perspective is that this specific check needs to happen before you decide how deep the apartments are.

The mistake I keep retelling happened in 2019. We had a unit with an awkward small bedroom, and we combined two spaces into one large primary bedroom. It looked better and the client approved it. What we missed was the glazing ratio: the larger room had the same window, so it dropped below the 8 percent requirement. We caught it during permit review and had to revise the plan back.

3. Lock the plumbing stacks before you finish the interior layout

An office building typically has a handful of wet columns. An apartment building has bathrooms and kitchens every few hundred square feet. If the interior designer and the architect are not solving this together from the first schematic, it will cost you.

The common trap is putting kitchens and bathrooms against exterior walls because it makes the renderings look light and open. In a conversion, that often means running waste lines horizontally across the slab to reach an existing stack. Those pipes need slope, and they steal ceiling height from the unit below. In 2021 I approved a plan that had kitchens along the window wall for seven floors. It was a good sales story and a bad plumbing layout. The coordinator asked one question—where does the waste stack go—and the redesign took another two weeks.

Now we mark the stacks and wet walls before unit plans go to the client. Kitchens and bathrooms can still be where the light is good, but only if there is a clean vertical path for the pipes. It is a constraint that, if ignored, multiplies construction cost.

4. Draw a full section through every repeated condition

Office buildings have vertical clearances that make conversion possible. But conversion eats that height from both sides: acoustic toppings, new sprinkler mains, ductwork, and fire-rated partitions all take space. The phrase that should worry you is almost enough.

The September 2022 RFIs came from exactly this. The corridor ceiling looked fine in the reflected ceiling plan, but we had not drawn the section where an existing transfer beam crossed above the corridor. The contractor caught the conflict in the field, not in the document set. Fixing it meant re-routing the duct and cutting the ceiling height in that area, which affected roughly a dozen floors.

If the same plan repeats on ten floors, draw the section through that repeated plan and through every place where something different passes overhead. A single accurate section is worth more than a hundred rendered perspectives.

5. Treat procurement lead time and schedule certainty as part of the design

The critical path in an office-to-residential conversion is rarely the design work. It is the items with long lead times: windows, entrance doors, elevators, fire-rated glazing, and sometimes prefabricated bathroom pods. By the time you have finished the drawings, it is already too late to order them without paying for expediting.

In March 2024, on a conversion with a fixed completion date, our client had two window bids. The lower one came with a maybe—delivery around the end of May. The other came with a guaranteed date, but cost about $24,000 more in expediting. The way I see it, that $24,000 was not paying for speed. It was buying certainty. If a window delivery slips by six weeks, the carrying cost on that project was more than $200,000. In that situation, the maybe is the expensive option.

I'm not saying every rush fee is worth it. If the schedule has room, take the cheaper option. But if a loan, permit, or occupancy commitment drives the date, make guarantee a line item in the budget. We have been burned twice by probably-on-time suppliers. Now we ask for delivery dates in writing and we pay when a date matters.

6. Build a full-scale mock-up of one unit before you replicate it across many floors

This is the step that gets cut first. It feels like an unnecessary construction cost, especially when the design is already in BIM. But a mock-up is not for checking the design; it's for checking the assumptions that the design is based on.

In an existing office building, the slab may not be level, columns may be out of plumb, and shafts can be two inches away from where the old drawings say they are. A full-scale mock-up of one bathroom, one kitchen wall, and a section of demising wall, built on the actual floor, exposes tolerance problems before the same detail has been repeated two hundred times.

On a recent project, the mock-up cost about $30,000. It caught a wall thickness conflict that would have appeared in every unit on six floors. Repeating the fix during construction would have cost several times the mock-up. If a physical mock-up truly is impossible, laser-scan the actual bays and have the general contractor walk the critical dimensions with you before you issue the final package. But honestly, the physical mock-up is worth it.

Two warnings that do not fit neatly into the checklist

First, treat the ground floor as its own project. An office lobby is not a residential lobby. Residents generate daily deliveries, package lockers, bike storage, trash, and strollers. We've seen conversions where the upper floors are efficient and the ground floor becomes a bottleneck that frustrates residents and management.

Second, get the hazardous materials survey into the schedule before construction budgeting. Older office buildings often have spray-applied fireproofing, sealants, or insulation that require abatement. You do not want to discover it after the demolition permit is issued, because abatement time does not compress well.

For full transparency: my project experience is based on U.S. conversions between roughly 80 and 300 units. A small 30-unit building or a concrete high-rise outside the U.S. can behave differently. I'm also not a structural engineer, so I can't speak to seismic retrofit questions. What I can speak to is the sequence that has saved our team from repeating expensive mistakes.

My biggest lesson after about fifteen conversions is that a checklist is not bureaucratic overhead. It is the reason I can sleep during construction. If the deadline is fixed and the budget is tight, you might think you don't have time for all six checks. That is exactly when you need them most. Trust me on this one—I learned it the expensive way.