Lease Side Letters in the UK: When Businesses Should Use Them

A lease side letter can look harmless, especially when it is sent over as a short follow up to your agreed heads of terms or just before completion. That is exactly why businesses get caught. A founder agrees to a rent concession in an email chain, assumes a side letter changes the lease forever, or signs a personal arrangement without checking whether it ends on assignment, renewal or default. Those mistakes can turn a useful commercial deal into a dispute about rent, rights to fit out, exclusivity or whether a promise is even enforceable.

For UK businesses, side letters are common in commercial leasing, but they need careful drafting. They are often used to give temporary rent relief, record a landlord concession, or deal with a practical issue that the parties do not want in the main lease. The detail matters. Here, we explain when a lease side letter makes sense, what it can and cannot safely do, the legal issues to check before you sign, and the mistakes businesses make when they treat a side letter like a casual admin document.

Overview

A lease side letter is a separate document that sits alongside a commercial lease and records an additional agreement between landlord and tenant. It is often useful where the parties want flexibility or confidentiality, but it is not a shortcut around careful drafting.

The main question is not whether a side letter is shorter than a lease. The real question is whether it clearly states who gets the benefit, when it starts, when it ends, and how it interacts with the lease if something goes wrong.

• Check whether the side letter is intended to vary the lease, waive a right, or grant a temporary concession.
• Confirm exactly when the arrangement starts and ends, and what triggers it to fall away.
• Make sure it says whether the benefit is personal to the current tenant or passes to an assignee, undertenant or guarantor.
• Review consistency with the lease, rent deposit deed, guarantee and any agreement for lease, licence for alterations or fit out works.
• Consider whether the wording could affect security of tenure, break rights, rent review positions or future negotiations.
• Ensure the document is properly executed and kept with the full lease pack, not buried in emails.

What Lease Side Letters in the When Businesses Should Use Them Means For UK Businesses

For most UK businesses, a lease side letter is a practical way to document a commercial concession without rewriting the whole lease. It is commonly used where the parties agree something specific, limited or personal that should sit beside the main lease rather than inside it.

That might happen before you sign a lease, during a difficult trading period, after fit out delays, or as part of an incentive package for taking space in a new development. In each of those founder moments, speed matters, but clarity matters more.

What is a lease side letter?

A lease side letter is a separate written agreement linked to an existing or proposed commercial lease. It usually deals with a discrete point, such as a rent free period, stepped rent, delayed enforcement, turnover reporting arrangement, fit out timetable, exclusivity wording, service charge cap or temporary consent.

It does not replace the lease. The lease remains the primary contract for occupation rights and obligations. The side letter sits alongside it and should say how the two documents work together.

When do businesses typically use one?

Businesses often use side letters where the commercial deal needs flexibility or a point is too sensitive to include in the lease itself. Common situations include:

• a landlord agrees reduced rent for the first six months while a tenant gets established
• rent concessions are offered during refurbishment delays or slow handover
• a landlord allows staged fit out works or delayed opening obligations
• the tenant receives a temporary waiver for an existing breach
• the parties agree a short term service charge cap or contribution arrangement
• a concession is intended to be personal to the current tenant and not available to a buyer or assignee

Landlords also use side letters where they want to preserve the headline lease terms for valuation, lending or estate management reasons, while still giving a limited concession to secure a tenant.

Why not just amend the lease?

A formal lease variation may be better where the change is permanent, structural or likely to affect third parties. A side letter is more commonly used for arrangements that are personal, temporary or conditional.

This distinction matters. If your concession is supposed to last only while your business occupies the premises, or only while rent is paid on time, a side letter can express that. If the arrangement changes the core legal estate in a lasting way, relying on a side letter can create uncertainty.

What kinds of terms belong in a side letter?

A side letter is usually best for a narrow issue with a clear commercial purpose. Terms often included are:

• temporary rent reductions or stepped rent arrangements
• short term deferral of rent or service charge payments
• personal concessions linked to the named tenant
• landlord consent to a specific use or operational point, subject to conditions
• timetables for opening, trading or completing works
• waivers of enforcement for a known issue, for a limited period

Businesses should be careful about using side letters for major changes to repair obligations, alienation rights, term length, break mechanics or core rights over the premises. Those points often need fuller contract drafting in the lease or a formal deed of variation.

Are side letters legally binding?

They can be, but only if drafted and executed properly. A side letter is not automatically less binding because it is shorter or labelled as a letter. The court will look at substance, not just title.

This is where founders often get caught. A short document may still create enforceable obligations, or it may fail because it is too vague, inconsistent with the lease, or lacks proper execution where a deed is required. Email wording can add more confusion if the formal documents do not match what was negotiated.

Legal Issues To Check Before You Sign

Before you sign a lease side letter, pin down whether it is a temporary commercial concession, a binding variation, or a one off waiver. If that is unclear, the document can create arguments about duration, enforceability and who benefits from it.

Is the concession personal or does it attach to the lease?

This is one of the most important drafting points. Many landlords intend a side letter benefit to be personal to the original tenant only. That means it should end if the lease is assigned, underlet, transferred within a group, or if occupation changes in some other way.

If the wording is unclear, a tenant may argue the benefit continues. If you are the tenant and the concession matters to your business model, you need to know whether it survives a restructure, investment transaction or future sale.

What triggers bring the concession to an end?

A side letter should clearly state the events that switch the concession off. Typical triggers include:

• late payment of rent or service charge
• breach of another lease covenant
• insolvency events
• assignment or underletting
• failure to complete fit out or open for trade by an agreed date
• expiry of a stated concession period

The drafting should also say whether the concession ends automatically or only after notice from the landlord. That practical point can decide whether a missed payment permanently removes a rent concession or whether the tenant has a chance to fix the breach.

Does the side letter conflict with the lease?

Conflicts between the lease and the side letter are a common source of trouble. If the lease says full rent is payable quarterly in advance, but the side letter allows monthly reduced payments for a period, the documents need to spell out which wording prevails for that period.

The same issue comes up with break clauses, guarantor obligations, rent deposit drawdown rights, repair obligations and permitted use. The side letter should not leave the parties piecing together the real deal from inconsistent clauses.

Could the document affect security of tenure or future lease rights?

Most side letters are not designed to alter statutory renewal rights, but wording around term, occupation rights, exclusivity, options or landlord discretion can have knock on effects. This is especially relevant if the premises are occupied under a protected business tenancy, or if the parties are also negotiating renewal terms.

If a side letter is trying to do more than record a concession, for example giving extra occupation rights or changing the parties' position on renewal, it needs close review. A short side letter is not a safe place for an unclear deal on long term property rights.

Do related documents also need changing?

A lease side letter often sits within a wider lease pack. Before you sign, check whether any related document also needs updating, such as:

• a rent deposit deed
• a guarantor or authorised guarantee agreement
• a licence for alterations
• a fit out agreement
• an agreement for lease
• existing correspondence that sets out incentive terms

If the side letter changes payment timing or conditions, a rent deposit arrangement may need to reflect that. If a guarantor is on the hook for rent, changing the concession without thinking about the guarantee can create a mismatch.

Should the side letter be executed as a deed?

The right execution method depends on what the document is doing. Some side letters are signed as simple contracts. Others may need deed formalities, especially where the document effectively varies a lease or deals with matters that are better formalised that way.

The point for business owners is simple: do not assume the side letter can be signed casually because it is short. Execution mistakes can weaken the enforceability of the arrangement at the exact moment you need certainty.

Can you rely on emails or heads of terms instead?

Not safely. Heads of terms are often non binding, and email negotiations rarely give enough certainty on all points. Before you spend money on setup or commit to a site, make sure the final side letter captures the exact concession, dates, conditions and consequences of default.

If something matters commercially, put it in the formal lease documents. A founder should not be proving a rent incentive from scattered email messages six months later.

Common Mistakes With Lease Side Letters in the When Businesses Should Use Them

The biggest mistake is treating a lease side letter like an informal note rather than a legal document with real consequences. Most problems come from assuming everyone shares the same understanding, when the wording does not actually say that.

Assuming a rent concession lasts for the whole term

A tenant may read a side letter as a long term discount. A landlord may intend it to be revocable on default or personal to the first tenant only. If the side letter does not clearly state duration and conditions, the parties can end up in dispute after a single late payment or a group restructure.

Before you sign a lease, ask a plain question: when exactly does this benefit end, and what events cause it to disappear?

Using a side letter for a major lease rewrite

Some businesses try to deal with core lease terms in a short side document because the transaction is moving quickly. That is risky. If you are changing repair liability, alienation rights, break dates, term length or major operational rights, a more formal variation may be the safer route.

A side letter works best when it deals with a confined issue. Once it starts acting like a second lease, the drafting risk rises sharply.

Forgetting about assignees, undertenants and group companies

Businesses often restructure after taking space. They may assign the lease to another group company, grant occupational rights to a trading entity, or plan a future sale. If the side letter concession is personal, those changes can switch it off.

That does not just matter for legal neatness. It affects property value, buyer due diligence and your ongoing occupancy costs.

Leaving default wording too broad

Many side letters say a concession ends on any breach of the lease. That sounds simple, but it may mean a minor technical breach removes a valuable rent arrangement. Businesses should look closely at whether the trigger is proportionate.

Sometimes the right drafting is tied only to material breach, payment default, or breach that is not remedied after notice. The correct position depends on the commercial deal and bargaining power.

Not checking lender, superior landlord or estate requirements

In some lease structures, the immediate landlord may have restrictions under a superior lease, finance arrangements or estate documents. A side letter that conflicts with those arrangements can create practical problems, even if the tenant did nothing wrong.

This can come up in shopping centres, serviced estates, mixed use developments and franchise style occupancy models. If the concession affects use, signage, turnover reporting or charging structures, wider estate rules may matter.

Failing to keep the side letter with the lease records

A surprising number of disputes start because the side letter is signed, scanned and then forgotten. Finance teams invoice the wrong rent. Asset managers enforce the lease without seeing the concession. New owners or managers inherit an incomplete file.

Keep the executed side letter with the lease, any agreement for lease, rent deposit documents and completion pack. Internal teams should know the operative position from day one.

Relying on vague drafting around fit out or opening obligations

Retail, hospitality and studio operators often need practical flexibility before opening. A side letter may help by softening opening dates or fit out milestones, but vague language creates more problems than it solves.

If works are delayed, the side letter should state:

• the revised timetable
• what counts as practical completion or opening
• whether rent starts during delay
• what evidence must be provided
• whether the concession falls away if the timetable is missed again

Without that detail, both parties may think they have protection when they do not.

FAQs

Is a lease side letter the same as a deed of variation?

No. A side letter is usually a separate agreement dealing with a specific point, often temporary or personal. A deed of variation is generally used where the lease itself is being formally changed in a more permanent way.

Can a side letter give temporary rent relief?

Yes, that is one of the most common uses. The wording should clearly state the amount of relief, payment dates, whether arrears accrue later, and what events end the concession.

Does a side letter automatically pass to a new tenant on assignment?

No. Many side letters are drafted to be personal to the original tenant. If you may assign the lease later, check this point before you sign.

Can a landlord withdraw a side letter concession at any time?

Not unless the side letter allows that. The landlord's right to end the concession depends on the wording, including any default triggers, notice requirements and end date.

Should email promises about lease incentives be included in the side letter?

Yes. If an incentive matters commercially, it should be captured in the final signed documents. Relying on informal exchanges creates avoidable uncertainty.

Key Takeaways

• A lease side letter is a separate document used to record a specific agreement alongside a commercial lease, often for temporary or personal concessions.
• It can be useful for rent relief, fit out timing, waivers and other limited arrangements, but it should not be treated as informal paperwork.
• The key legal issues are whether the arrangement varies the lease or simply grants a concession, when it ends, and whether it is personal to the current tenant.
• Businesses should check how the side letter interacts with the lease, guarantee, rent deposit and any related property documents.
• Clear drafting on default, assignment, notice and duration is essential before you sign a lease or spend money on setup.
• Where the change affects core lease rights or long term property interests, a more formal variation may be needed.

If you want help with rent concession drafting, lease variation issues, assignment risk, and fit out timing terms, you can reach us on 08081347754 or team@sprintlaw.co.uk for a free, no-obligations chat.