Cascade mechanisms in a section 106 agreement
A cascade sets out what happens when the agreed affordable tenure cannot be placed with a registered provider. The triggers, the evidence and the floors decide whether it still delivers homes.
A cascade in a section 106 agreement is a fallback. It names the affordable tenure you have agreed to deliver, then a second tenure if the first cannot be placed, then usually a payment in lieu, and it sets out what has to be proved before the obligation drops a step. Get the drafting wrong and a permission granted on the basis of social rented homes can end up delivering a cash sum and nothing on site.
Registered providers do not always buy. Grant programmes close, a provider pulls out after board review, stock transfers stall. The cascade exists so that a scheme is not frozen while the parties renegotiate a deed that took nine months to complete in the first place.
What a cascade clause does
Section 106 of the Town and Country Planning Act 1990 lets an authority secure obligations that restrict development or require a payment. An affordable housing obligation usually fixes three things: the number of affordable units, the tenure split between them, and the mechanism for transferring them to a provider at a stated price.
A cascade adds a conditional route through those three. It typically reads as a sequence. Offer the units as social rent at the agreed transfer value. If no registered provider contracts within a defined marketing period, offer them as affordable rent. If that fails, offer shared ownership or a discounted market product. If none of that works, pay a commuted sum calculated by a formula in a schedule.
The point of the drafting is that each step is triggered by evidence rather than by the developer's preference. A cascade without an evidential trigger is a licence to pick whichever option costs least.
The triggers that move you down a step
The trigger is almost always failure to contract after a marketing exercise. What varies, and what is worth arguing over at drafting stage, is how that failure is defined.
A tight clause will specify the marketing period in months, the price at which the units must be offered, the list of providers who must be approached, and the form of evidence supplied to the council. A loose clause says the developer must use reasonable endeavours and leaves everything else open. Reasonable endeavours clauses generate disputes about whether the offer price was realistic, which is exactly the argument the cascade was meant to avoid.
Other triggers appear in particular schemes. Loss of grant funding is one, though authorities resist it because grant availability is outside the council's control and inside the provider's. Building safety remediation costs on a converted block is another. Where a trigger relies on a matter of judgement rather than a fact, expect the council to require its own written confirmation before the step takes effect.
Marketing periods and evidence of no interest
Six months is a common marketing period for a first tenure and three months for each step after. There is no national standard, so check what your authority's affordable housing supplementary planning document says before assuming a figure.
The evidence pack usually includes the marketing particulars, the date each provider was approached, the price offered, the responses received and a short statement from the developer's agent explaining why the offers were declined. Councils scrutinise the price more than anything else. If the units were offered at a transfer value well above what providers in that area were paying for comparable stock, the authority will say the marketing was not genuine and refuse to accept the trigger.
Set the transfer price by reference to a published basis rather than a number in the deed. A percentage of open market value for each unit type, assessed at the time of the offer by an independent valuer appointed jointly, survives market movement better than a fixed pound figure agreed three years before completion.
Caps, floors and the value the council keeps
The risk the council is managing is that each step down the cascade reduces the public benefit while the permission stays the same. Floors are the answer. A floor fixes a minimum below which the obligation cannot fall.
Common floors include a minimum number of on-site affordable units regardless of tenure, a minimum proportion of the units that must be rented rather than intermediate, and a commuted sum calculated so that the council can deliver at least the same number of homes elsewhere. The last of these is where the arithmetic gets contested, because a sum that only reflects the difference between market and affordable transfer value will not buy an equivalent home on the open market.
| Step | Typical trigger | Usual safeguard |
|---|---|---|
| Social rent to affordable rent | No provider contracts within the marketing period | Unit numbers and mix unchanged |
| Rented to intermediate | Second marketing period expires | Minimum rented proportion retained |
| On site to commuted sum | All tenures unplaced | Sum indexed and calculated to replacement cost |
| Payment deferred | Phased delivery | Payment secured by bond or land charge |
Caps work the other way and protect the developer. A cap on the commuted sum, or a longstop date after which the obligation converts automatically, gives a lender something to underwrite. Whether you get one depends on how much the council needs the scheme.
Approval at each step
Most cascades require the council's written confirmation before the next step is reached. That confirmation is not a planning decision and it is not appealable, which makes the timescale for giving it worth negotiating. A clause that says the council shall confirm within 20 working days of receiving the evidence, failing which confirmation is deemed given, protects the programme without removing the council's role.
Watch the interaction with other obligations. If the biodiversity or infrastructure contributions are triggered by occupation and the affordable units are the ones being built last, a cascade dispute can hold up the whole phase. Reading the deed as a single programme rather than a set of independent schedules catches that early. The same discipline applies to a commuted sum clause that sits in a different schedule and uses a different index.
Drafting a cascade that still delivers
The failure mode is a cascade that reaches the commuted sum too easily, because the money arrives years later and buys fewer homes than the site would have produced. Councils that have been through this once tend to draft the next agreement much harder.
Four drafting habits reduce the risk. Fix the transfer value by formula rather than by number. Name the providers to be approached, or require approaches to every provider active in the authority area on a published list. Require the evidence pack to be submitted in a defined form. And put a date on the whole cascade so that it cannot run indefinitely alongside a partly built scheme.
On the developer side, the argument that carries weight is deliverability rather than value. If the site cannot support the agreed tenure, a viability case supported against a defensible benchmark land value is the route, not a cascade drafted loosely enough to swallow the obligation.
An illustrative structure
This is illustrative and not a form of words to copy. A schedule might define the Affordable Housing Units and the four tenures in order of priority. It would then require the developer to market the units to registered providers for six months at a price equal to a stated percentage of open market value, assessed by a jointly appointed valuer, and to submit a marketing report in a defined form. On the council's written confirmation, the obligation converts to the next tenure. A final paragraph sets the floor: no fewer than a stated number of rented units on site in any event.
An Affordable Housing Statement submitted with the application should describe the intended tenure split and say plainly whether a cascade is proposed and why. Officers dislike finding one for the first time in a draft deed.
What to check before you sign
Read the definition of the marketing period and confirm it starts on a date you control. Check whether the transfer price is fixed or formulaic. Check the floor. Check that the commuted sum formula is indexed and that the index is named. Check who confirms each step and how long they have. Then check that nothing in the cascade conflicts with a phasing or occupation trigger elsewhere in the deed.
This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it, and have any draft agreement reviewed by a solicitor experienced in planning obligations.
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Frequently asked questions
What is a cascade mechanism in a section 106 agreement?
It is a fallback sequence in an affordable housing obligation. The deed names a first-choice tenure, then a second, then usually a payment in lieu, and sets out the evidence needed before the obligation moves down a step. The trigger is normally failure to contract with a registered provider after a defined marketing period at a stated price.
How long is a typical marketing period before the cascade steps down?
Six months for the first tenure and three months for each later step are common, but there is no national figure. The period is a matter for the deed and for the council's affordable housing supplementary planning document. Check what your authority expects before assuming a duration, because a short period invites the argument that marketing was not genuine.
Can a cascade end in a commuted sum instead of homes on site?
Yes, and most cascades do provide for that as the final step. Councils usually resist it because the money buys fewer homes later than the site would have produced. Expect a floor that keeps a minimum number of rented units on site, and expect the sum to be calculated by an indexed formula rather than fixed as a number in the deed.
Is a cascade the same as a review mechanism?
No. A review mechanism revisits viability at a later date and can increase or decrease the obligation based on actual costs and values. A cascade changes the tenure or form of the obligation when the agreed product cannot be placed. A scheme can carry both, and the drafting should make clear which takes effect first.
Who confirms that a cascade step has been triggered?
Almost always the local planning authority, in writing, after receiving a marketing evidence pack. That confirmation is not a planning decision and carries no right of appeal, so negotiate a response period into the deed. A clause giving the council 20 working days, after which confirmation is deemed given, protects the programme without removing the council's role.
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.