Unilateral undertakings against bilateral agreements
Both instruments are made under section 106 and both bind successors. The choice turns on whether the council needs to promise anything in return, and on how fast you need the deed completed.
A unilateral undertaking is a planning obligation the landowner signs alone. A bilateral agreement is signed by the landowner and the council together. Both are made under section 106 of the Town and Country Planning Act 1990, both bind successors in title, and the choice between them usually comes down to whether the council needs to promise anything in return.
That is the practical test. If the obligations all run one way, a unilateral undertaking section 106 instrument will do the job and will normally do it faster. If the council has to covenant to repay unspent money, to adopt something, to transfer land or to act in a defined way, you need an agreement.
What each instrument is
Section 106(9) sets the formal requirements for any planning obligation. It must be by deed, state that it is a planning obligation for the purposes of section 106, identify the land and the relevant interest in it, identify the person entering into it, and identify the local planning authority by whom it is enforceable.
A unilateral undertaking meets those requirements with the owner as the only party. The authority is named as the body that can enforce it, but it does not execute the document and it makes no promises.
A bilateral agreement adds the council as a party, and frequently adds others: a mortgagee, a registered provider of affordable housing, a management company, a highway authority. Once the council is a party it can give covenants of its own, and that is the substantive difference between the two.
Where a unilateral undertaking is the right tool
Four situations account for most of the undertakings that get used.
- A single financial contribution with no conditions attached to how the council spends it.
- A small site where the council's legal team has no capacity and the delay is the main risk.
- An appeal, where the authority will not negotiate an agreement because it is defending a refusal.
- A householder or minor application where the drafting cost of an agreement is out of proportion to the obligation.
The programme advantage is real. An undertaking can be drafted by your solicitor, executed by your client and dated on the day the council asks for it, without waiting for a council solicitor to pick up the file. On a small scheme that difference is often several weeks, and the gap between a committee resolution and the decision notice is usually where it is felt.
Why authorities push for a bilateral agreement
Councils resist undertakings for reasons that are mostly sensible.
They cannot give their own covenants in a unilateral document, which matters where an obligation needs a repayment clause for money not spent within a defined period, a mechanism for releasing the obligation once it has been satisfied, or a commitment about how a contribution will be applied. A developer who wants any of those needs the council on the deed.
They also lose control of the drafting. An authority reviewing an undertaking it did not write has to satisfy itself that the definitions work, that the triggers are unambiguous, that the land is correctly described and that everyone with an interest has executed it. That review takes officer and legal time whether the document is unilateral or not, so the saving is smaller than applicants expect on anything complicated.
Where the obligation involves affordable housing the position is close to settled in favour of an agreement, because a registered provider will need to be a party or to have the benefit of mortgagee protection provisions, and the tenure and nomination arrangements set out in the Affordable Housing Statement have to be translated into covenants with defined remedies.
Monitoring fees and legal costs
Both instruments can include a covenant to pay the council's monitoring fee, and both can include a covenant to pay its legal costs. An undertaking does not escape either.
What changes is the negotiation. Where the council is drafting a bilateral agreement, it will normally ask for an undertaking as to its legal costs before it starts work, and those costs accrue whether or not the deal completes. With a unilateral undertaking the legal cost sits mostly on your side, and the council's charge is limited to reviewing the document and the monitoring fee.
Monitoring charges are constrained. The Community Infrastructure Levy Regulations limit what an authority can charge for monitoring planning obligations, and a fee has to relate to the actual monitoring the obligation requires rather than to a flat percentage. Ask what the fee covers and over what period.
Enforceability against successors in title
This is the point clients worry about unnecessarily. A planning obligation is enforceable against the person who entered into it and against anyone deriving title from that person, under section 106(3), regardless of which instrument was used.
Registration is what makes it work in practice. The obligation is registered as a local land charge, so it appears on a search and a purchaser's solicitor will find it. A unilateral undertaking is no weaker on this than an agreement.
Two drafting points matter more than the choice of instrument. Everyone with a legal interest in the land should execute, or the document should contain mortgagee exclusion provisions, because an obligation that does not bind the lender is worth much less on an enforcement sale. And the obligations should be conditional on the grant of the permission and on implementation, so a permission that is never built out does not leave the covenants running.
Using an undertaking at appeal
At appeal a unilateral undertaking is the standard mechanism, because there is no realistic prospect of the council executing a deed while resisting the scheme.
The practical requirements are unforgiving. The document must be completed, signed and dated, not a draft, and it must reach the Inspectorate by the deadline in the appeal timetable. An inspector will then test each obligation against the statutory tests: whether it is necessary to make the development acceptable in planning terms, directly related to the development, and fairly and reasonably related in scale and kind.
Include only obligations that pass those tests. An undertaking loaded with contributions the council asked for but cannot justify invites the inspector to disregard them, and an obligation that fails the tests cannot be taken into account. The planning case for each obligation belongs in the Planning Statement rather than in the deed itself.
One tactical point. Where the council's position on contributions is unclear, some appellants submit two undertakings, one with a wider set of obligations and one narrower, each conditional on the inspector's findings. It works, but it has to be drafted carefully, and a badly drafted alternative creates ambiguity about what has actually been promised.
The drafting timetable and engrossment
Whichever instrument you use, the sequence is the same and the delays sit in the same places.
- Agree heads of terms before the committee date, not after.
- Identify every party with a legal interest at the outset, including mortgagees, option holders and any management company.
- Obtain title and check that the land description in the deed matches the red line on the application.
- Produce the first draft, and offer to do it on the council's own template where a bilateral agreement is needed.
- Circulate the engrossment for execution, allowing for corporate signing requirements and any lender consent.
- Date the deed, and confirm the council has what it needs to issue the decision notice.
Where a site straddles two authorities, both need to be named as enforcing bodies and the obligations apportioned, which is one of the complications covered in our note on cross-boundary sites and applications to two councils. Once completed, the obligation becomes part of the public record alongside the permission, as set out in our note on the statutory planning register.
One honest uncertainty. Whether an authority will accept a unilateral undertaking on a scheme of any complexity is a matter of local practice rather than law, and some councils decline as a matter of policy. Ask at pre-application, because the answer changes your programme by weeks.
PlanScope drafts the supporting documents an application with obligations needs, structured to UK planning policy and ready for review. Enter your postcode on PlanScope to see which reports your site is likely to need.
This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
What is the difference between a unilateral undertaking and a section 106 agreement?
A unilateral undertaking is executed by the landowner alone, with the authority named only as the body that can enforce it. A bilateral agreement adds the council as a party, which means it can give covenants of its own. Both must satisfy section 106(9), be made by deed and identify the land, the interest and the enforcing authority.
When should I use a unilateral undertaking?
Where the obligations all run one way. A single financial contribution, a small site where council legal capacity is the main risk, an appeal where the authority will not negotiate, or a minor application where the drafting cost of an agreement is out of proportion. Anything needing a council covenant requires an agreement.
Is a unilateral undertaking enforceable against a future owner?
Yes. Section 106(3) makes a planning obligation enforceable against the person who entered into it and against anyone deriving title from them, whichever instrument was used. Registration as a local land charge means a purchaser's solicitor will find it. Execution by mortgagees, or mortgagee exclusion provisions, is what actually needs attention.
Can I submit a unilateral undertaking at appeal?
Yes, and it is the normal mechanism because the council will not execute a deed while defending a refusal. It must be completed, signed and dated rather than a draft, and it must reach the Inspectorate by the deadline. The inspector will test each obligation against the statutory tests before taking it into account.
Do I still pay a monitoring fee on a unilateral undertaking?
Usually yes. Both instruments can include a covenant to pay the authority's monitoring fee and its legal costs. The Community Infrastructure Levy Regulations limit what can be charged for monitoring, and the fee should relate to the monitoring the obligation actually requires rather than to a flat percentage of the contribution.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.