Conversion Is an Engineering Question First

Homes need daylight and natural ventilation in places offices do not. A deep office floor plate, common in buildings from the 1970s onward, can leave a large central core of space too far from a window to be a habitable room. Designers can carve light wells or use the centre for storage and services, but each response costs area and money.

This single factor rules out a large share of office stock before anything else is considered.

An office has a few wet areas stacked around the core. An apartment building has a kitchen and a bathroom in every unit. Running new soil, waste and water services to dozens of units means new risers, penetrations through slabs, falls for drainage and often raised floors to carry pipework, which eats into floor-to-floor height.

Most conversions are a plumbing project with a building attached. That is where the cost, the risk and the delay concentrate.

Ventilation follows the same pattern. Each unit needs extraction and fresh air, and the central systems that served open office floors are rarely suited to many small separate dwellings.

Conversion is growing, but from a small base, and pipeline is not delivery.

England is the instructive case. Class MA, the permitted development right for converting commercial premises to homes, was relaxed on 5 March 2024 to remove the 1,500 m² floorspace cap and the vacancy test. Completions under it are rising, but total office-to-residential output under permitted development has fallen. Removing planning friction did not remove the engineering.

Practitioners put figures on the constraints this paper describes. Residential conversion typically needs one to three feet of additional floor-to-floor allowance for the services, plumbing and sprinklers that homes require. Floor plates deeper than roughly 80 to 120 feet from core to facade struggle to give every room daylight. Where cities have used subsidy to close the gap, the sums are large: Chicago has committed more than $315 million of tax increment finance to six projects totalling 1,765 homes.

None of these is a rule. They are the screens a feasibility study runs first, and they explain why the conversion candidates that work tend to be older, narrower buildings with generous floor heights, rather than the deep-plan offices with the highest vacancy.

Residential use changes the fire strategy. Each unit becomes its own compartment, escape routes change, and sprinkler and detection requirements may increase. Acoustic separation between units, which offices never needed, must be built in. And the facade, often designed for an office that was cooled more than heated and occupied by day, may need upgrading for thermal comfort, condensation control and openable windows.

In older buildings these works sit alongside the issues described in What Age Conceals: hazardous materials in the way of new penetrations, concrete condition, and facade materials that may not meet current expectations.

A first engineering screen for conversion can be run on most office buildings before any design work. Floor-plate depth from window to core, against the depth at which habitable rooms can still get daylight. Floor-to-floor height, against the space needed for new services and acceptable ceiling heights. Structural grid and floor capacity. Number and location of risers, and where new wet stacks could run. Facade type, openability and condition. Fire stair and lift provision against residential needs. Hazardous materials likely to be disturbed.

Buildings that pass most of these screens are worth designing. Those that fail two or more are usually better candidates for refurbishment in their existing use or for the redevelopment question discussed in When the Building is Not the Asset.

None of this means conversion is a bad idea. Narrow plates, generous floor-to-floor heights, good structural grids and facades with openable or replaceable glazing can convert well. Many older, smaller office buildings fit that description better than the large post-war towers.

The point is order. Conversion should start with an engineering reading of the building, which then informs the planning and financial case, rather than the other way round. The analysis in When the Building is Not the Asset shows the other side of the same decision: sometimes the honest answer is that the land, not the building, carries the value. New retrofit methods could loosen the screen described here; it is a current reading of what is practical, not a permanent one.