Empty offices are everywhere. Hybrid working emptied floors that once held hundreds of desks. Converting them into flats looks like a neat solution. The structure is already there. The location is often central. The planning route can be quicker.
So why do so many conversions run over budget?
The answer is usually hidden in the ceilings, risers and plant rooms. Plumbing and electrical systems built for an office are a different animal from those a block of flats needs. Developers who treat building services as a line item, rather than a design problem, tend to find out the hard way.
Here’s where the money goes, and how to see it coming.
Offices and Homes Use Buildings in Completely Different Ways
An office is built around shared facilities. A typical 1980s office block might have one toilet core per floor, a small tea point, and a single incoming electrical supply feeding the whole building.
A home needs all of that, in every unit.
Picture a four-storey office with one washroom core per floor. Convert it into 24 flats and you suddenly need:
- 24 kitchens with sinks, dishwashers and washing machine connections
- At least 24 bathrooms, often more with en-suites
- 24 separate hot water systems or heat sources
- 24 consumer units, each with its own metered supply
- 24 individual smoke and heat detection systems
That’s not a refurbishment. That’s a new building services installation sitting inside an old shell. And it rarely shows up properly in early cost plans.
Drainage: The Problem Gravity Won’t Solve for You
Water runs downhill. That sounds obvious, yet it causes more redesigns on office conversions than almost anything else.
Office toilets are clustered around a central core with one or two soil stacks. Flats need bathrooms and kitchens spread across the floor plate. Every one of those needs a route back to a stack, with enough fall to work.
A 100mm waste pipe typically needs a fall of around 1:40. Run that pipe 8 metres from a bathroom on the far side of a flat and you lose 200mm of height. Where does that 200mm come from?
Developers usually face three options:
- Raise the floor. Workable if the floor-to-ceiling height allows it, but it eats into headroom that buyers and renters care about.
- Add new soil stacks. This means cutting through floor slabs, which brings in structural engineers and fire stopping costs.
- Use pumped drainage. Macerators and small pumping stations solve awkward layouts, but they add maintenance, noise and a point of failure.
Some layouts that look perfect on a drawing simply can’t be drained without one of these. Finding that out after the architect has fixed the floor plans is expensive. It’s far cheaper to have a plumber walk the building at concept stage.
Hot Water and Heating: One Boiler Room Won’t Cut It
Many older offices were heated by a central plant room and cooled by air handling units. Some had no meaningful hot water system at all beyond a few point-of-use heaters at sinks.
Residential use demands far more. A family shower might draw 10 to 12 litres per minute. Multiply that across dozens of flats at 7.30am on a weekday and the demand is nothing like an office.
The decision on heating and hot water drives cost across the whole project. The main routes are:
- Communal heating with heat interface units in each flat. Efficient at scale, but the pipework, metering and billing arrangements need careful design.
- Individual heat pumps or electric systems per flat. Simpler to bill, but they put serious pressure on the electrical supply.
- Individual gas boilers. Increasingly out of step with energy targets and harder to justify on new homes.
Each choice affects the next. Pick electric heating and your electrical upgrade gets bigger. Pick communal heating and your plumbing scope and long-term management costs grow.
There’s also legionella to consider. Office buildings that have sat empty for months often have stagnant water in tanks and dead legs. The HSE’s guidance (ACoP L8) recommends stored hot water at 60°C and distributed hot water reaching at least 50°C. Old, oversized pipework and redundant runs make that hard to achieve. Stripping out dead legs and flushing the system should be planned, not discovered.
For projects in the East of England, specialists like Royal Flush Plumbing can survey existing pipework early and flag which runs can be reused and which need to go.
The Electrical Supply Is Probably Not Big Enough
This is the one that catches developers out most often.
An office is supplied as a single commercial customer. Often there’s one large three-phase supply, a main switchroom and distribution boards on each floor. The loads were lighting, computers and perhaps some small power for kitchens.
Homes behave differently. Think about what a modern flat draws at peak time:
- An induction hob
- An electric oven
- A washing machine or washer dryer
- A heat pump or electric heating
- Possibly an electric shower
- Possibly an EV charger in the car park
Now multiply that across every unit. Even with diversity applied, the combined demand can far exceed what the existing supply was designed for.
If the building needs more capacity, the developer has to apply to the Distribution Network Operator (DNO). That can mean a new substation, new cables in the street, or both. Costs can run into tens of thousands of pounds, sometimes far more. Lead times can stretch to many months.
Have you checked the available capacity before buying the building? Many developers haven’t. A quick load assessment and an early enquiry to the DNO can change the viability of the whole scheme.
Every Flat Needs Its Own Installation
Beyond supply capacity, each home needs a complete, independent electrical installation. That covers:
- Separate metering so each household pays its own bills
- A dedicated consumer unit with RCD protection to BS 7671
- New final circuits for sockets, lighting, cooking and heating
- Landlord circuits for communal lighting, door entry, lifts and emergency lighting
- Fire detection designed for residential use, not office use
Office wiring is almost never reusable for flats. Circuits are laid out around open-plan desks, often running in floor boxes and ceiling voids that the new layout will remove. Most conversions strip out the existing installation and start again.
Fire alarm design deserves special attention. Offices are typically covered by a single building-wide system. Flats usually need detection within each dwelling, plus systems covering escape routes and communal areas. Getting the interface between these right matters. A poorly designed system can lead to frequent false alarms or, worse, gaps in protection.
Hidden Surprises in Older Buildings
Office stock from the 1960s to the 1980s often brings problems you won’t see on a walkthrough.
Common finds include:
- Asbestos in ceiling tiles, pipe lagging and riser linings, which must be surveyed and dealt with before services work begins
- Aluminium or undersized cabling that can’t be safely extended
- Lead or galvanised steel pipework that’s corroded internally
- Riser shafts too small for the extra cables, pipes and ducts a residential scheme needs
- Poor or missing fire stopping around existing service penetrations
Each of these adds cost and time. None of them can be priced accurately from drawings alone. A proper intrusive survey by the trades who’ll do the work is worth every penny.
Why the Budget Gap Happens
So why do these costs get missed?
Often it’s because the early appraisal treats the building as a shell and prices services per square metre using a generic rate. That works for a new build. It doesn’t work for a conversion, where the existing building dictates the solution.
Other common causes:
- Building services engineers brought in after the layout is fixed
- No DNO enquiry until the design stage
- Assuming existing risers and drainage can be reused
- Underestimating the number of wet rooms per unit
- Too little contingency for unknowns behind walls and ceilings
On conversions, a contingency of 10 to 15% on services is far more realistic than the 5% often used on new builds.
How to Get Ahead of the Problem
The fix is simple, even if it isn’t cheap. Bring plumbing and electrical expertise in at the start.
Before you commit to a layout, you should know:
- How much electrical capacity is available and what an upgrade would cost
- Where soil stacks can go and how far each wet room sits from one
- Whether risers can take the extra services
- What heating and hot water strategy suits the building and the budget
- What hazardous materials will need removing first
Answer those five questions early and most of the nasty surprises disappear.
In Norfolk and Suffolk, firms such as Eastern Electrical carry out load assessments, condition reports and full rewires on commercial buildings. That makes it easier to understand what an existing installation can and can’t support before a single wall comes down.
The Building Services Decide Whether the Scheme Works
Office-to-residential conversions can be a smart way to create homes and bring tired buildings back to life. But the shell is the easy part.
The real work happens in the pipes, cables, risers and switchrooms. Those systems decide how many flats you can fit, where the kitchens and bathrooms go, and whether the numbers stack up.
Treat plumbing and electrical design as the foundation of the scheme, not a late add-on. Your budget will thank you.
