If you’re looking at commercial space in the UK, the single most important number on the lease is probably wrong. Agents are sometimes known to inflate square footage, and this should be borne in mind as a weakness of the data, according to PropertyData. That means the rent you think you’re agreeing to could be based on a figure that doesn’t match reality. I’ve watched this trip up business owners for years, and it’s one of the first things I check when someone tells me they’ve found a good deal.
Rents are expressed as an annual rate per square foot based on the Net Internal Area (NIA) of the property. That standardised measurement allows for direct comparison between different properties and locations, but only if you know what measurement is actually being used. The problem is that not every landlord or agent uses the same one. If you don’t know the difference between NIA and Gross Internal Area (GIA), you could end up paying for space that doesn’t exist. Here’s what you actually need to know.
What Net Internal Area actually means for your rent
When you see a rent quoted at £50 per square foot, that square foot is almost certainly based on Net Internal Area. NIA is the usable floor area of a property — it excludes toilets, corridors, stairwells, lift shafts, and structural walls. If your lease uses Gross Internal Area instead, you’re paying for walls and shared spaces you can’t actually use. That’s a difference that can add 10% to 20% to your effective rent without you realising it.
What I’d do: ask the agent or landlord to confirm in writing which measurement standard the quoted square footage uses. If they hesitate, that’s a red flag. You can also check the Valuation Office Agency data on PropertyData, which uses NIA as its benchmark, to see if the figure lines up with comparable properties in the area.
Why getting the square footage wrong costs you real money
In the City of London, the average prime rent rose 40% year-on-year to £130.80 per sq ft in Q1 2026. That means a 500 sq ft discrepancy — easily hidden in an inflated floorplan — would cost you an extra £65,400 per year. That’s not a rounding error; that’s a significant chunk of your operating budget.
Consider this scenario: you’re looking at a 2,000 sq ft office quoted at £100 per sq ft. You think your annual rent is £200,000. But if the actual NIA is only 1,700 sq ft because the agent included a shared corridor and a storage cupboard, your real rent per usable square foot jumps to £117.65. You’re paying for space you can’t use, and you’ll never get that money back.
The situation is even more acute in the West End, where prime rents average £165 per sq ft and the top rent hit £201 per sq ft at 77 Grosvenor Street. At those levels, even a small measurement error becomes a six-figure problem. I’ve seen businesses sign leases based on agent-provided floorplans, only to discover during fit-out that their actual usable space is 15% smaller than advertised. By then, the lease is signed and the rent is fixed.
What I’d do: before you sign anything, get a chartered surveyor to measure the space independently. It costs a few hundred pounds and can save you tens of thousands. If the landlord refuses access for a measurement, walk away. That’s not a negotiation tactic — it’s a warning sign.
Three mistakes that cost tenants thousands
Trusting the agent’s floorplan without verification
PropertyData explicitly warns that agents are sometimes known to inflate square footage. This isn’t a rare occurrence — it’s a known weakness of the data. The problem is that most tenants don’t have the tools to verify the figure. They see a floorplan, assume it’s accurate, and sign the lease. By the time they realise the space is smaller than advertised, the rent is locked in.
The fix is straightforward: hire a surveyor to measure the property using the RICS Code of Measuring Practice. This is the industry standard, and it gives you a legally defensible figure. If the landlord’s figure doesn’t match, you have grounds to renegotiate the rent before you sign.
Confusing NIA with GIA
Gross Internal Area includes everything inside the external walls — toilets, stairwells, lift shafts, and structural columns. If your lease uses GIA instead of NIA, you’re paying for space you can’t use. The difference can be substantial, especially in older buildings with thick walls and awkward layouts.
I’ve seen leases where the rent was calculated on GIA, and the tenant didn’t notice until the first service charge bill arrived. By then, the lease terms were already set. The fix is to check the lease wording before you sign. If it says “Gross Internal Area” or doesn’t specify a measurement standard, ask for it to be changed to NIA. Most landlords will agree if you ask early enough.
Ignoring the service charge per square foot
The rent per square foot is only half the story. Service charges are also calculated on the same measurement, and they can add 20% to 50% to your total occupancy cost. If the square footage is inflated, your service charge is inflated too — and you’re paying that every year for the entire lease term.
What I’d do: ask for a breakdown of the service charge and check that it’s calculated on the same measurement as the rent. If the landlord uses different measurements for different charges, that’s a red flag. You can read more about managing commercial service charges to understand what you should be paying for.
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| Location | Grade A rent per sq ft | Grade B rent per sq ft |
|---|---|---|
| Mayfair / St James’s | £165 – £201 | £70 – £103 |
| City of London | £130.80 | £70 – £90 |
| Soho | £165 | £80 – £100 |
| Covent Garden | £142.50 | £75 – £95 |
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How to verify square footage and negotiate the right rent
Get an independent measurement before you negotiate
Hire a chartered surveyor to measure the property using the RICS Code of Measuring Practice. This gives you a legally defensible figure that you can use in negotiations. The cost is typically £300 to £600, depending on the size of the property. Compare that to the potential cost of an inflated measurement — £65,000 per year in the City — and it’s one of the best investments you can make.
Once you have the surveyor’s figure, compare it to the agent’s floorplan. If there’s a discrepancy of more than 2%, ask the landlord to explain. If they can’t provide a satisfactory answer, use the surveyor’s figure as the basis for your rent negotiation. Most landlords will adjust the rent rather than lose a tenant over a measurement dispute.
Check the lease for the measurement standard
The lease should specify which measurement standard is being used. If it doesn’t, ask for it to be added. The standard for commercial property in the UK is NIA, as defined by the RICS Code of Measuring Practice. If the lease uses GIA or doesn’t specify, that’s a negotiation point. You can ask for the rent to be reduced to reflect the difference, or for the lease to be amended to use NIA.
If you’re dealing with a complex lease or a landlord who’s being difficult, it’s worth getting a tenant landlord lawyer to review the terms. They can spot measurement-related clauses that you might miss and advise on how to negotiate them.
Factor in the supply crunch when you negotiate
The market is tightening fast. In the City Core, there’s only 1.3 years of Grade A supply left. In Mayfair, it’s even tighter at 0.7 years. That means landlords have the upper hand in negotiations, especially for prime space. But that doesn’t mean you have no leverage. If you can demonstrate that the square footage is wrong, you have a concrete basis for a rent reduction that the landlord can’t easily dismiss.
What I’d do: start your search 12 to 18 months before your planned move. That gives you time to verify measurements, negotiate terms, and walk away if the deal isn’t right. For larger requirements over 100,000 sq ft, lead times can be up to four years. The earlier you start, the more leverage you have.
Consider the future of the space you’re renting
With only 1.3 years of supply under construction across the whole UK, competition for Grade A space is only going to intensify. Above-inflation rental growth is forecast for all prime UK markets in 2026. That means the rent you agree today could look like a bargain in two years — but only if the square footage is accurate.
If you’re looking at a longer lease, make sure there’s a rent review clause that allows for adjustments based on market conditions. And if you’re considering a serviced office versus a traditional lease, remember that serviced offices typically charge a fixed monthly fee that includes the measurement — so you don’t have to worry about inflated square footage, but you also don’t have the same control over the space.
What’s the difference between NIA and GIA? ▾
Can I measure the space myself? ▾
What if the landlord refuses to adjust the rent? ▾
How do I know if the agent’s floorplan is accurate? ▾
Does the service charge use the same measurement? ▾
Getting the square footage right is the single most important step in renting commercial space. It affects your rent, your service charge, and your ability to plan your layout. A few hundred pounds spent on a surveyor now can save you tens of thousands over the life of the lease. If this was useful, you might also want to read key tips for renting office space in the UK.
Sources and Further Reading
London commercial property: costly leasing mistakes — A practical guide to the most common errors tenants make in the London market, from measurement disputes to break clause timing.
UK Real Estate Market Outlook 2026: Offices. CBRE, 2026.
London Office Rent Market Overview. K2 Space, Q1 2026.
Understanding Commercial Property Rents. PropertyData, 2026.
