Converting a 1960s office floor plate versus a single story retail box: which starting point actually works better

Retail box, and it's not close for me. Trenching a slab is a known cost per foot. Cutting window openings in block is also a known cost per opening, lintels and all. Both of those you can price in an afternoon.

The office one has a problem you can't price, which is that 60 feet of depth means a chunk of your floor is unrentable and you don't know how much until you've drawn units three times. I've never made money on a building where the design was the risk.

I'd take the office and I think @joe_delgado5 is underrating the core. Existing risers, existing elevator, and existing stairs mean the three most expensive and most code-sensitive elements are already there and already approved for some occupancy. In the retail box you're adding a second stair for any second floor you build, and if you stay single story you've got 18,000 sf of ground floor apartments which is a specific and not always strong product.

The deep plate is real. Two-bedroom units run deeper, den layouts absorb some of it, and the interior can become storage and mechanical. It's a solvable geometry problem.

Also the office building on a pad with windows on four sides is much better than the mid-block office with two blank party walls, which is the version that actually doesn't work.

A couple of terms in here that are worth having plain, since both buildings come down to the same handful of physical facts.

Floor plate depth is the distance from the exterior window wall to the middle of the building. Apartments need daylight and ventilation, and most residential codes limit how far a habitable room can sit from a window. So a 60 foot depth means the middle band of the building often can't be bedrooms or living rooms. It becomes closets, bathrooms, hallways, or unused space you still paid for.

Slab on grade means the ground floor sits directly on a concrete slab poured on the dirt, with no basement or crawl space underneath. To add plumbing you saw-cut and trench the slab. It's dusty and it takes time, and it lets you put a bathroom almost anywhere. In a building with framed floors over occupied space below, moving plumbing is much harder.

The light and air requirement @wendy_park48's third poll option points at is the one to check first in either building. How much window area a habitable room needs, and whether courtyards or light wells count toward it, is set by the code your jurisdiction has adopted, and adoptions differ from state to state and city to city. Ask the building department which edition applies before you draw anything.

Neither building type is generally better. The office gives you infrastructure and takes away geometry. The retail gives you geometry and takes away daylight. Which trade is cheaper is a per-building answer.

Voted for the local code option. Take two retail boxes of the same shape in two different towns, where one code allows light wells to count toward the window requirement and the other reads it much more narrowly. Same building shape, completely different deal.

Nobody has mentioned the roof. On an 18,000 sf single story you own a lot of roof per unit, and if you are cutting light wells you are cutting the roof too, which means new flashing on every one and a warranty conversation. The office building has a third of the roof per square foot of unit. It is a real line item people skip, even if it is not enough on its own to decide the question.