Mobile signal is poor inside most modern office buildings because the materials that make a building energy-efficient also block radio waves, while the mobile networks serving it are designed primarily to cover streets, not interiors. The signal that reaches your desk has usually travelled from a mast hundreds of metres away, then fought its way through coated glass, concrete and insulation, losing most of its strength on the way. Around 80% of mobile data traffic is generated inside buildings (Ericsson Mobility Report), yet almost all of it is carried by infrastructure built for the outdoors.
If you are reading this because calls drop in meeting rooms, payment terminals time out, or people drift towards windows to send a message, the causes below will be familiar. The useful part is understanding which of them apply to your building, because that determines what will actually fix it.
The building itself is the main culprit
Every improvement in building energy performance over the past two decades has made life harder for mobile signals. Low-emissivity glass, the coated glazing used to keep heat in during winter and out during summer, contains a thin metallic layer that reflects radio waves almost as effectively as it reflects infrared. Reinforced concrete, steel frames and foil-backed insulation do the rest. A building designed to BREEAM or equivalent standards is, from a radio perspective, a well-sealed box.
This is why the problem is worst in the newest and best buildings. An older office with single glazing and brick walls often has tolerable indoor signal simply because it leaks. A recently refurbished Grade A office frequently has worse mobile performance than the building it replaced, which surprises landlords who assumed that a premium specification meant premium connectivity.
5G makes the physics harder, not easier
A reasonable assumption is that newer network generations will solve the problem. The opposite is true. 5G networks use higher radio frequencies than 4G, and higher frequencies carry more data but penetrate solid materials less effectively. A 5G signal that performs superbly on the pavement outside can be unusable ten metres inside the facade. As operators shift more capacity to these higher bands, the gap between outdoor and indoor performance widens rather than closes.
The practical consequence is that a phone indoors often falls back to older, lower-frequency layers of the network. Users see full bars but experience slow data, because those legacy layers have limited capacity and everyone in the building is sharing them.
Mobile networks are designed from the outside in
Operators plan their networks around outdoor coverage maps. Masts are positioned to cover streets, transport corridors and open areas, and indoor coverage is largely a by-product: whatever happens to make it through the walls. No operator can economically build dedicated systems inside every commercial building, so the default state of any given office is that nobody is responsible for the signal inside it.
This also explains why coverage varies by operator within the same building. Each operator’s nearest mast sits in a different direction and at a different distance, so one tenant’s phones may work adequately while another’s barely function, depending on nothing more than which side of the building their office faces and which network their company contract happens to use.
Why some floors and rooms are worse than others
Signal strength inside a building follows a predictable geography. Spaces near windows on upper floors facing a mast perform best. Performance degrades towards the building core, where lift shafts, stairwells, risers and plant rooms add layers of concrete and steel. Basements, underground parking and lift cars are usually dead zones, since no meaningful outdoor signal reaches them at all. Meeting rooms suffer disproportionately because they are often placed in the core, precisely where people gather expecting to make calls.
What does not fix it
Wi-Fi calling helps individual users with compatible phones and well-configured devices, but it is a workaround rather than a solution. It depends on the office Wi-Fi being fast, secure and available to that person, which excludes visitors, contractors and most guests the moment they walk in. Payment terminals, alarm systems and IoT devices that rely on mobile connectivity gain nothing from it.
Signal boosters, which capture outdoor signal and re-broadcast it indoors, can genuinely help in small spaces where the underlying problem is penetration rather than capacity. Their limitation is that they can only amplify what already exists outside: if the local mast is weak or congested, the booster faithfully reproduces that weakness indoors. We have written a separate article on when a booster is enough and when it is not.
What actually fixes it
The structural answer is to bring the mobile capacity inside the building rather than hoping outdoor signal survives the journey in. Dedicated indoor mobile infrastructure distributes 4G and 5G through small cells placed throughout the property, connected back to the operators over fibre, so the building has its own capacity independent of conditions outside. Built as a shared system, a single installation serves all operators, which means every phone in the building works regardless of whose contract it is on. Fewer than 5% of commercial buildings currently have dedicated indoor mobile infrastructure (Proptivity analysis), which is why poor indoor signal remains the norm rather than the exception.
For a property owner, the relevant question is no longer whether indoor coverage matters, tenants have settled that, but how a specific building performs today and what level of infrastructure it warrants. That varies with size, construction, tenant density and how the building is used.
Find out how your building performs
If you want to see where your building stands, our free coverage check uses real-world mobile measurement data to show how signal performs at your address, and our team prepares a full indoor coverage assessment at no cost and with no obligation. It is the quickest way to move from suspecting a problem to knowing exactly what you are dealing with.