Office to residential: when prior approval is the route

Converting offices to flats is attractive for an obvious reason: the building already exists, so the shell is paid for and the scheme can move faster than new build. The route through the planning system is often faster too, because some conversions can go through a prior approval process rather than a full application. Deciding which route a particular building takes is a decision to make early, and it belongs in planning and design coordination rather than being left to the architect to discover.

Whether the prior approval route is available at all depends on the building, the location, the planning history and the local authority, including whether the council has removed the right in that area. None of that can be assumed from another scheme, so it has to be confirmed with the authority and with a planning adviser for the specific site before anything is committed.

Prior approval is a narrower question

A full planning application asks whether the development is acceptable in principle and in detail. A prior approval application asks a narrower question: given that the change of use is permitted in principle, are the specific matters the authority is allowed to consider satisfactory.

That narrowing is the advantage. The authority is not weighing the principle of residential use on the site, so the debate is smaller and the determination period is usually shorter. It is also the limitation: the matters the authority can consider are set out, and anything outside them is not up for negotiation in your favour either.

The practical consequence is that prior approval is not a lighter version of a planning application. It is a different application, with a different evidence base, and a scheme designed for one does not simply transfer to the other.

What the council still assesses

A prior approval application is still assessed, and it can still be refused. The matters typically examined include transport and highways impact including parking and servicing, contamination risk, flooding, noise from existing commercial uses nearby, and the adequacy of natural light in the habitable rooms being created.

Each of those can require a report, and each report can change the design. A noise assessment may dictate the glazing and ventilation strategy. A daylight assessment may rule out the deepest parts of a floorplate as bedrooms. A contamination review may require investigation of the ground even though you are not building foundations.

The exact list and the weight given to each item depend on the authority and on current national policy, which changes over time. The reliable approach is to ask the council what it will expect for that building and to commission the reports before the layout is fixed, not after the application has gone in.

Where the right may not be available

There are several reasons a particular building may not qualify. The council may have removed the right in that area. The building may be listed or in a designated area with its own controls. Its existing lawful use may not be the one the route requires, or the evidence of that use may be thin. There may be requirements about the period the building has been vacant, its size, or its location.

Each of those is a question of fact about your building and your borough, and the answers are obtained from the planning record and the authority rather than from general advice. Where the position is genuinely unclear, a conditional purchase structure is usually cheaper than an assumption, a point we make about every pre-exchange unknown in seven site risks to check before you exchange.

The route does not change the build cost

A faster consent does not make the conversion cheaper. Office floorplates were designed for one use and residential standards are a different set of requirements. The cost usually sits in the same places: compartmentation and fire strategy, acoustic separation between dwellings, new drainage runs through a structure that has none where you need them, replacing or upgrading the facade for thermal and ventilation performance, lift and core alterations, and an entirely new services installation on a residential basis.

Deep plans create a second problem. Where the distance from the facade to the core is large, part of the floorplate may be unusable as habitable space, which reduces the saleable area without reducing the cost of the shell. That is a cost plan question and it needs testing on measured areas before the land price is agreed, as set out in feasibility and cost planning.

Measuring an existing building accurately is itself a piece of work, and the record drawings are rarely a safe basis for it. We cover the method in measuring existing buildings.

When a full application is the safer choice

Prior approval suits a conversion that stays within the existing envelope and does not need external change. Once the scheme wants more than that, the picture changes. Adding storeys, altering the external appearance, extending, changing the footprint or combining the conversion with new build will usually take the scheme outside the narrow route and into a full application.

There is also a case for a full application even where prior approval is available. A full permission can give greater certainty on the detail, allows the design to be optimised rather than constrained, and produces a consent that a funder and a future purchaser can read without caveats. Where the scheme is marginal on saleable area, the extra months may buy a better building.

That trade-off is a commercial decision with a programme and finance cost attached, so it should be modelled rather than argued. A longer consent period is more interest and more holding cost, which is exactly the kind of comparison development management exists to put numbers against.

What to settle before you commit

Four things, in order. Whether the route is available for that building in that borough, confirmed with the authority. What reports the council will expect, and what they are likely to constrain. What the conversion costs once fire, acoustics, drainage, facade and services are measured rather than assumed. And what saleable area actually survives the daylight and layout constraints.

Those four answers turn a conversion from an opportunity into a scheme. They also tell you what to pay for the building, which is the only number the deal cannot be revised after. The design coordination that follows, and the cost planning and estimating behind it, are what keep the consent and the budget pointing at the same scheme.