Comparable evidence in a London office rent review is the inventory of recent, verified, arm's-length lettings a surveyor uses to argue what your floor is worth. It is also the single largest line item in the difference between paying market rent and paying whatever your landlord's surveyor proposes. Get it wrong on a 10,000 sq ft City office and you pay an extra million or two over a five-year cycle. Get it right and the negotiation flips. This article is a working guide for the CFO, COO, or office director on the receiving end of a rent review memo, written from the tenant side, and grounded in the commercial rent review framework.

What a rent review actually is, in plain English

A rent review is a clause in your commercial lease that lets the landlord reset your rent at fixed intervals, usually every five years on a 10- or 15-year lease. Most office reviews are upward-only: the rent can rise or stay flat, but it cannot fall, no matter what the market has done since you signed.

The reset is to the open market rent on the review date. That is the rent a willing landlord would have agreed with a willing tenant if your floor had been offered, vacant, on the open market that day. It is a hypothetical exercise. It is also the basis on which a real number, paid in real money, lands in your service charge debit every quarter for the next five years.

The mechanics run in four steps. First, the landlord serves a rent review memorandum proposing a new figure. Second, you respond with your own figure, usually after a surveyor has analysed comparable lettings in your submarket. Third, the parties negotiate, and most reviews settle here. Fourth, if there is no agreement, the dispute goes to a third party — either an arbitrator or an independent expert, depending on the wording of your lease.

The single question that decides the number, in steps two through four, is: what is the open market paying for floors like yours, right now? Comparable evidence is the answer.

What comparable evidence is, and what it costs you to get it wrong

Comparable evidence is a schedule of recent, verifiable office lettings in your submarket, on properties as close to yours as possible, with the headline rent stripped back to what the parties actually paid once all the incentives are amortised. That last number is the net effective rent, and it is the only honest comparator. The headline rent that runs in the property press is, more often than not, a marketing figure.

The cost of getting it wrong is direct and quantifiable. Take a 10,000 sq ft City office. The landlord proposes £130/sq ft against a passing rent of £85/sq ft. Your surveyor, on a thin set of comparables, argues £110/sq ft. The arbitrator awards £120/sq ft.

The difference between a thinly evidenced case and a strongly evidenced one is a million pounds over the cycle. On larger floors or longer terms, the number scales linearly. This is why comparable evidence is the central piece of work in any contested rent review, and why landlord-side surveyors invest in it as a full-time professional discipline.

The hierarchy: what counts as evidence and what does not

The rulebook is the RICS Professional Standard Comparable Evidence in Real Estate Valuation, first issued October 2019 and reissued as a Professional Standard in April 2023. It binds every RICS member, on both sides of the table, and any arbitrator or independent expert applying it.

The Standard sorts evidence into three tiers, with a clear ranking inside the top tier.

Within Category A the weight runs from strongest to weakest. Open-market lettings between two strangers transacting at arm's length sit at the top. Agreed rent reviews and lease renewals are next, discounted because the parties are not at large in the market. Third-party awards come below those, both because they reflect a tribunal's opinion rather than a market transaction, and because of the rule in Land Securities plc v Westminster City Council on admissibility. Asking terms on currently marketed property sit at the bottom because nothing has actually been agreed.

The Standard then requires every comparable to pass seven tests. It must be:

1. Comprehensive — drawn from multiple data points, not a single deal.
2. Very similar to the subject property in location, age, size, specification and lease terms.
3. Recent — representative of the market on the review date.
4. The result of an arm's-length transaction between unconnected parties.
5. Verifiable — confirmed with the parties to the deal, not lifted untested from a database.
6. Consistent with local market practice.
7. Drawn from a market with underlying demand, not a distressed or forced sale.

If a comparable fails any of these, it does not belong on the schedule. Six of the seven are properties of the deal itself. The seventh, verifiable, is the one where most tenant cases collapse, because the data needed to verify a comparable lives in private files the tenant cannot reach.

Devaluation: turning a headline rent into a number your finance team can defend

Devaluation is the surveyor's discipline of stripping a transaction back to the net effective rent, so two lettings on different lease terms can be compared like for like. It is unglamorous. It is also where contested rent reviews are won and lost.

The standard adjustments compound:

• Rent-free periods are amortised over the term certain (typically to first tenant break) and deducted from the gross rent.
• Capital contributions to fit-out are added to the rent-free pot and amortised on the same basis.
• Stepped rents are smoothed to a constant equivalent.
• Floor level, in towers, attracts a premium of roughly £0.50 to £2.50 per square foot per floor.
• Quantum discounts adjust for floorplate size: larger floorplates command lower £/sq ft.
• Fit-out condition (Cat A shell, Cat A+ plug-and-play, fully fitted) is rebased against a common standard.
• Term length, break dates, user clauses, alienation, and the repairing covenant all flex the headline against the standard FRI baseline.

Then there is the measurement point that costs more tenant cases than any other. IPMS 3 replaced NIA as the RICS office measurement standard from 1 January 2016. IPMS 3 produces an area roughly 2% to 5% larger than NIA on the same office, because it includes internal columns and structural elements that NIA strips out. Mix NIA and IPMS comparables on a single schedule without rebasing and every line is wrong by 2% to 5%. Many pre-2016 database transactions are still on NIA. Your CFO's first verification job is to confirm every comparable on the schedule is measured on the same basis as the subject.

A worked example you can run on your own deal

Take a recent 5,000 sq ft City letting at a headline rent of £85 per square foot, on a 10-year FRI lease with a tenant-only break at year five, 24 months rent-free, and a £40 per square foot capital contribution.

Devalued over the term certain (five years to break):

• Gross rent: £85 × 5,000 × 5 = £2,125,000
• Less rent-free: 24 months at £35,417/month = £850,000
• Less capital contribution: £40 × 5,000 = £200,000
• Net rent received: £1,075,000
• Divide by 25,000 sq-ft-years: £43.00/sq ft net effective

Devalued over the full ten years (assuming the break is not exercised):

• Gross rent: £4,250,000
• Less the same incentive package: £1,050,000
• Net rent: £3,200,000
• Divide by 50,000 sq-ft-years: £64.00/sq ft net effective

The same headline £85/sq ft deal is evidence at £43 to one surveyor and £64 to another, depending on the assumption about the break. That is a 50% spread on a single comparable. Tenant-side surveyors argue term-certain, because no rational tenant would have signed if they were obliged to commit beyond the break. Landlord-side surveyors argue full-term, because the break is an option, not an obligation. The arbitrator picks one.

If you cannot model a comparable to the level above, in your finance system, with the incentive package broken out and the term-certain assumption stated, you do not have a comparable. You have a press release.

What London looks like in 2026, and why a £93/sq ft gap matters to you

Central London office rents are now more polarised than at any point in the last two decades. The numbers below come from the Savills Central London Office Market Watch, April 2026 unless otherwise stated.

The Grade B collapse is the most important figure on the page for any tenant in secondary stock. City Grade A averages £130.80/sq ft. City Grade B averages £37/sq ft. The gap is £93/sq ft. Across a 10,000 sq ft floor and a ten-year cycle, that is £9.3 million.

If your building is now legitimately Grade B office stock — 1990s or early-2000s specification, average EPC, no recent capex by the landlord — and the schedule presented to you is built on Grade A comparables, the entire schedule is wrong. The right comparables for your building exist; they sit in a different part of the database and they tell a different story.

The cleanest illustration of the City step-change is Proskauer Rose at 8 Bishopsgate. The firm took 60,000 sq ft in 2023 at £86/sq ft. In August 2025 it took an additional floor in the same building at roughly £150/sq ft. Same tenant, same building, two years apart, a 74% increase on the headline. That is the shape of the spread the comparable schedule has to measure. It also tells you the comparable book your landlord built two years ago is already out of date.

Why you cannot pull this evidence yourself

The verification problem, not the database problem, is what locks tenants out of the comparable evidence market. Three reasons.

Headline rents are public; everything else is private. The £100/sq ft letting that runs in the trade press the day it completes does not come with the 24-month rent-free, the £50/sq ft capital contribution, the break flexibility, or the side letters. Those are agreed between principals' agents and held in confidence. Without them, the headline rent is not Category A evidence. It is Category B noise dressed up as a transaction.

The subscription databases are enterprise-priced and require trained interpretation. CoStar Lease Comps, EG Interactive, and Radius are licensed to professional users on per-site, per-seat contracts. A one-month trial in the hands of a finance director cannot reliably distinguish an arm's-length open-market letting from an intra-group re-gear, a distressed sale-and-leaseback, or a sweetheart deal between connected parties. Yet the difference between those four transaction types is the difference between Category A evidence and inadmissible noise.

The Land Registry redacts the financials. Registered commercial leases are public records, but the rent, incentives, capital contributions, and side letters are routinely redacted before registration. The Registry confirms the lease exists. It does not tell you what it cost.

That leaves direct enquiries with the letting agents on the other side of each deal, ninety days at a time. Those relationships are how a real comparable schedule is built. It is also the work the RICS Standard contemplates when it requires the source to be interrogated and verbal evidence tested with both sides. Tenants, as a rule, do not have those relationships. Landlords' agents do. The asymmetry is structural, and it is the reason the same proposed rent settles at materially different numbers depending on whose evidence is admitted.

The four cases your landlord's surveyor will cite at you

You do not need to read them in full. You do need to know they exist, because every well-fought review touches at least one of them.

Land Securities plc v Westminster City Council [1993] 1 WLR 286. A previous arbitrator's award is not direct evidence of the open market rent in your review. It is that arbitrator's opinion. Your landlord's surveyor cannot import another tribunal's reasoning to justify the number proposed for you, although he can give his own opinion of value. Push back when an award is presented without the underlying transactional facts.

Segama N.V. v Penny Le Roy Ltd (1984) 1 EGLR 109. Lettings that complete after your review date can still be used as evidence, provided nothing has happened in the interim to disconnect them from the market on the review date. This matters for tenants whose review date sits in a quarter of low transaction volume; the comparables that surface in the months after the review are still admissible, and they often help.

Cornwall Coast Country Club v Cardgrange Ltd [1987] 1 EGLR 146. Only information that would have been available to prospective lessees in the open market on the review date counts. Private information held by the actual parties — your trading data, the landlord's internal modelling, confidential side letters on other deals — is irrelevant. This is the basis of the willing lessor / willing lessee hypothetical that runs through every rent review clause.

Bunyan (VO) v Acenden Ltd [2023] UKUT 17 (LC). A rating case, but the methodology travels. The Upper Tribunal rejected the proposition that a Cat A-condition building lets for the same rent as a Cat B-fitted building. Fit-out is rentalisable and must be adjusted out before two comparables compare like for like. Cite this when your landlord's surveyor presents a fitted comparable letting against your shell-condition space without adjustment.

The duty of the surveyor signing the schedule sits on top of all four. Under CPR Part 35.3, the surveyor's primary duty is to the tribunal, not to the client. A schedule that reads like advocacy rather than analysis is, in principle, inadmissible. Jones v Kaney [2011] UKSC 13 removed expert witness immunity, so the surveyor putting the schedule together has personal liability on the line. Both points apply equally on a renewal under section 34 of the 1954 Act.

What to do when a rent review memorandum lands on your desk

Five steps, in order.

1. Read the rent review clause first. Identify the assumptions, the disregards, and whether disputes go to arbitration or independent expert. The hypothetical lease set out in your clause is the frame inside which every comparable then has to be applied. This is a 30-minute job that frequently saves a 30-week argument.
2. Audit the landlord's schedule against the seven RICS tests. Each comparable must be comprehensive, very similar, recent, arm's length, verifiable, consistent with practice, and from a real market. Mark each line of the schedule against each test. Any line that fails a test comes off, or comes down in weight.
3. Demand the deal terms behind every headline rent. Rent-free period, capital contribution, term length, break date, fit-out condition, service charge cap. Without these, no comparable can be devalued and no schedule has evidential weight. The landlord's surveyor has these numbers already. Make him share them.
4. Run the devaluation on both bases. Term-certain and full-term. Show the spread. The difference between the two is your negotiating range, and pinning the landlord to one or the other early is half the work.
5. Build a counter-schedule of two to four comparables you can verify. A short book of strong evidence beats a long book of weak evidence in every forum. The comparables you cannot stand up under cross-examination will be the ones the arbitrator strikes out.

The single fact that changes the negotiation is whether you have someone in the market who can verify the deals on the schedule and produce a counter-book the landlord's surveyor cannot dismiss. Headline rents are free. The work to convert them into the comparable evidence an arbitrator will rely on costs money, and the cost is a fraction of the gap between the schedule on the landlord's side and the rent your business should be paying.

Sources

RICS standards and guidance
- RICS Professional Standard: Comparable Evidence in Real Estate Valuation (April 2023) — the binding standard governing how surveyors must select, weight, and present comparable evidence
- RICS: Surveyors Acting as Arbitrators in Commercial Property Rent Reviews, 9th edition — arbitration procedure and the admissibility rules that govern comparable schedules in arbitration

Legislation
- Landlord and Tenant Act 1954, section 34 — statutory basis for renewal rent; the open market rent assumptions and disregards that carry over into most rent review clauses by analogy
- Civil Procedure Rules Part 35 — Experts and Assessors — the expert witness rules under which a surveyor's comparable schedule is presented to an arbitrator or court; primary duty to the tribunal

Market data
- Savills Central London Office Market Watch, April 2026 — source for City average prime £130.80/sq ft (+40% YoY), West End prime £165/sq ft, rent-free incentives (10.3% of total lease value), and Grade A supply data
- Savills Central London Office Market Watch, Q2 2025 — source for City Grade B average £37/sq ft (−27% YoY)
- Cushman & Wakefield London Office MarketBeat Q4 2025 — source for Grade A supply data: City Core 1.1 years, Mayfair 0.7 years, St James's 1.4 years

Case reports and news
- Bisnow: New York legal practice shatters City rent record at 8 Bishopsgate (August 2025) — Proskauer Rose expansion floor at ~£150/sq ft, versus same tenant's 2023 floor at £86/sq ft
- Pilcher London: World's highest ever office rent, 30 Berkeley Square (2020) — Mercury asset management; £277.50/sq ft world record

Case law
- Land Securities plc v Westminster City Council [1993] 1 WLR 286 — arbitration awards as opinion, not market evidence
- Segama N.V. v Penny Le Roy Ltd (1984) 1 EGLR 109 — post-review-date lettings admissible where market is continuous
- Cornwall Coast Country Club v Cardgrange Ltd [1987] 1 EGLR 146 — the willing lessor / willing lessee construct; only publicly available information relevant
- Bunyan (VO) v Acenden Ltd [2023] UKUT 17 (LC) — fit-out condition must be adjusted out before Cat A and Cat B lettings are compared
- Jones v Kaney [2011] UKSC 13 — removal of expert witness immunity; personal liability of the surveyor signing the comparable schedule

Law Commission
- Law Commission: Business Tenancies — The Right to Renew — ongoing review of the 1954 Act; second consultation expected spring 2026; relevant to the renewal rent framework underpinning comparable evidence use