Executive summary
Using Ookla coverage data, we measured indoor mobile coverage across 280 of Greater London’s most sought-after offices, all of them Grade A and Grade B. Fewer than one in ten deliver reliable indoor coverage, and nearly half have a problem serious enough that large parts of the building can’t hold a usable mobile signal.
The Grade A label makes no difference: those buildings are no better connected than Grade B, and occupiers are just as likely to end up walking to a window to take a call. If anything, the features that define a modern, high-quality building seem to count against it. Larger buildings tend to perform worse, as do those with higher energy ratings, and steel and concrete frames trail older masonry. Age offers no advantage either: offices completed in the last five years are no more reliable than ones built before 2000.
Occupiers feel the gap every time a call drops in a lift or a stairwell, and much of London’s premium stock sits on the wrong side of it.
Reliable mobile coverage is becoming part of what makes an office worth working in, and much of London’s premium stock has fallen behind on it.
Why indoor mobile coverage matters in commercial offices
Most of the mobile data we use is generated indoors. Industry estimates put the figure at around four-fifths, and inside an office that share is higher still. People expect to walk into a building and stay connected without thinking about it, and they notice quickly when they cannot.
Modern buildings are getting harder for a signal to reach. The coated, energy-efficient glazing that keeps heat in also blocks much of the signal coming from the masts outside, and denser, better-sealed facades add to it. 5G has made this worse, not better, because the higher frequencies it runs on are weaker at getting through walls.
The outdoor signal buildings used to borrow from nearby masts is fading indoors just as people expect more of it. Older buildings often did better by chance, because their materials let more signal through. A new, sustainable, highly specified building can be one of the worst places in the city to take a phone call.
Very few commercial buildings have anything purpose-built to fix this. A widely cited estimate is that fewer than one in twenty has dedicated indoor mobile infrastructure. The rest depend on a signal that the building itself is increasingly designed to keep out.
The buildings we looked at and how we measure
The analysis covers 280 office buildings across Greater London, split fairly evenly between Grade A and Grade B. Every building is a sizeable commercial office, starting at around 50,000 square feet and running up past a million.
The sample spans the full range of the market. It includes buildings from before 2000 through to ones completed or refurbished in the last five years, energy ratings from unrated stock up to BREEAM Outstanding, and the main construction types found across the city. That spread is what lets us look at how coverage changes with each of these features, rather than reporting a single citywide average.
We used real-world signal readings taken inside each building during ordinary daily use, not a one-off survey, and combined them across the major operators. That reflects what a typical occupier actually experiences, rather than how one operator performs on a good day.
We don’t report the technical signal values, which mean little outside the industry. Every reading is sorted into one of three plain categories, a strong signal, a usable one, or a struggling one, and each building is judged on how much of it falls into each.
Across the whole sample, only 7% have reliable connectivity
9 in 10 of London’s most attractive offices have coverage they can’t count on
Across the whole sample only 7% are Reliable and 49% are Poor
Across the whole sample, only 7% are Reliable. Just under half, at 49%, are Poor. The remaining 44% are Patchy. Put another way, more than nine in ten of London’s most attractive offices have at least some indoor coverage they cannot count on. A building is rated Reliable when struggling readings are rare and a usable signal is available almost everywhere. It is Patchy when usable coverage is the norm, but noticeable dead spots appear. It is Poor when a large share of the building cannot hold a usable signal. Throughout this report, a building with a coverage problem means one that falls into the Poor category.
Reliable
A usable mobile signal almost everywhere inside. Dead spots are rare.
Patchy
Coverage works in places, but noticeable dead spots appear across the building.
Poor
Large parts of the building struggle to hold a usable signal. A real problem.
What the coverage data shows
The buildings that come off worst are the large, new, highly rated ones the market prices as the best, not the older or cheaper stock.
Do Grade A offices have Grade A connectivity?
The Grade A label is meant to signal the best of the market: prime location, top specification, the kind of building that commands the highest rents. It says nothing about whether you can use your phone inside.
Grade A buildings are Poor for indoor coverage in 49% of cases, the same share as Grade B. The two grades are almost indistinguishable, with only 8% of Grade A buildings rated Reliable against 7% of Grade B. Whatever the Grade A premium buys, a connected workplace is not part of it. For occupiers paying top rents on the strength of the label, that is a real gap between expectation and experience.
Graph: Indoor coverage by building grade
49% of Grade A offices have a poor indoor coverage rating, identical to Grade B. The premium label buys no connectivity advantage.
Why are London’s greenest buildings the worst connected?
You might expect the most carefully designed, highly rated buildings to perform best. On coverage, the opposite is true.
BREEAM Outstanding buildings, the top sustainability rating, are the worst performers in the sample: 54% rated Poor, against 48% of the lower-rated and unrated stock. The high-performance glazing and sealed facades that earn a building its environmental rating are the same things that shut out a mobile signal. A building can hold heat brilliantly and still be hopeless for a phone call.
Graph: Indoor coverage by energy-efficiency (BREEAM) rating
Are newer offices better connected? No.
Sort the buildings by age and the same thing shows up. Newer is not better connected.
Offices finished or refurbished in the last five years are Poor for coverage 47% of the time. Buildings from before 2000 do slightly better at 43%. The worst of the lot is the 2000 to 2009 vintage, at 58%. There’s no improvement over time, and the oldest stock has a slight edge if anything, because its materials let more signal through. For a market that assumes new means better, that’s an awkward result: the newest, priciest space isn’t fixing the problem and often makes it worse.
Graph: Indoor coverage by building age (year built or last refurbished)
How does building size affect indoor coverage?
Of everything we looked at, coverage tracks size most closely. Small offices under 100,000 square feet do best, 30% Poor and a healthy 16% Reliable. It falls away fast as buildings grow. In the 250,000 to 500,000 square foot range, 64% are Poor and just 2% Reliable. The biggest buildings stay deep in problem territory. Bigger buildings have more floor area sitting far from any window and any outside mast, so more of the building falls into a dead zone.
64 % of offices between 250,000 and 500,000 square feet have poor indoor coverage. Bigger buildings, bigger problem.
Graph: Indoor coverage by building size (floor area)
Which construction materials block mobile signal most?
Traditional masonry holds up best, 37% Poor. Steel frames are worse at 51%, reinforced concrete worst at 57%. The structural materials that make big, efficient buildings possible are also the ones a signal struggles to pass through.
Graph: Indoor coverage by construction material
Masonry
Steel
Reinforced Concrete
The coverage gap in context
The buildings most at risk share a profile: large, recently built or refurbished, highly energy-rated, and framed in steel or concrete, which is to say the buildings the market treats as premium. The features that justify the highest rents are the same ones that undermine the everyday experience of working inside.
| Building feature | Coverage problem | vs London average (49%) |
|---|---|---|
| Largest buildings (250k to 500k sq ft) | 64% | Well above average |
| Built between 2000 and 2009 | 58% | Above average |
| Reinforced concrete frame | 57% | Above average |
| BREEAM Outstanding rated | 54% | Above average |
| Steel frame | 51% | Around average |
| Grade A | 49% | At the average |
| Built before 2000 | 43% | Below average |
| Masonry construction | 37% | Below average |
| Smallest buildings (under 100k sq ft) | 30% | Well below average |
Older and smaller buildings tend to score better for a simple physical reason: thinner walls and plainer glass let more signal through. It is a property of how they were built, not something anyone designed for connectivity, and it is not something a flagship asset can count on. The fix is straightforward: fit a building with indoor mobile infrastructure built for it and properly managed, and the coverage problem goes away whatever the building is made of. Connectivity is now part of how buildings are judged, alongside the measures the market already takes for granted. The buildings that fix it deliver what occupiers expect; the ones that don’t get noticed for it.
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About this research
This analysis was produced by Proptivity, an indoor mobile infrastructure provider operating across Europe.
It draws on independent, real-world mobile signal measurements taken inside 280 Grade A and Grade B office buildings across Greater London, combined across the major operators and matched to building characteristics from CoStar, including grade, floor area, year of construction or last refurbishment, energy-efficiency rating and construction type.
Coverage ratings in this report (Reliable, Patchy, Poor) are Proptivity’s own plain-language classification, designed to describe the experience of a building’s occupiers rather than to report technical signal values.
Published June 2026.
For media enquiries or the underlying methodology, contact Proptivity at info@proptivity.com.
Based on Proptivity’s analysis of Ookla® Cell Analytics™ data from May 2025 to May 2026, matched to building characteristics from CoStar®. All rights reserved.