Key Takeaways
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Air extraction voids below stairs can make a standard lift pit unworkable, even when the rise itself is modest.
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The real design question is not just pit depth. It is how much of the under-stair volume can be occupied without harming airflow, maintenance access, or smoke extract performance.
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Sesame Access solves this by adapting the lift to the building services constraint, not automatically forcing the building services to move.
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On this type of project, split-depth design, independent step design, and accurate volumetric pit modelling are often more important than the nominal rise.
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Drainage still matters. If a pit sits above or beside a service void, coordination with drainage strategy, sump layout, and long-term maintenance is essential. This aligns with Sesame’s existing guidance on lift pit sump pump best practice and accessibility lift pit drainage design.
Introduction
A short rise at an entrance can look simple on plan. Two steps, a modest level change, and a clear desire to make the entrance accessible.
But when there is an active air extraction space below those stairs, the accessibility question changes completely.
Now the design team is not simply choosing a lift. They are trying to protect airflow, preserve service access, respect structure, and still create an entrance that works properly for wheelchair users. This is exactly the kind of problem Sesame Access is good at solving.
What makes this page different from other Knowledge Hub articles is its focus on the relationship between the lift and the air extraction void beneath it. This is not a general hidden-lift overview. It is a concept-stage engineering guide for projects where MEP constraints are driving the lift geometry.
Why This Is a Specialist Problem
Sesame Access has spent more than two decades engineering bespoke lifts for constrained and architecturally sensitive buildings. In projects like this, the difficulty is rarely the rise alone. The difficulty is how the lift has to coexist with the building conditions that are already there.
An active extract zone beneath a stair is one of those conditions. Standard product thinking usually breaks down because a normal pit assumption does not answer the real question: how much void volume must remain, how can the lift avoid blocking the service zone, and how can the design remain maintainable for the long term?
That is why this work has to be approached as bespoke engineering, not catalogue selection.
What Makes Air Extraction Below Stairs So Difficult?
When a building uses the area beneath the stair or landing as an extract plenum, ventilation void, or smoke relief space, the lift pit is competing with something the building already relies on.
In the meeting, the design issue was stated very clearly:
“They’re trying to work out what volume out of that pit beneath is going to be taken so that they can then calculate all the extract.”
That is the right question.
In this sort of scheme, the lift is not only taking physical depth. It is also taking volume from a service zone that may already be part of the building’s environmental strategy.
This creates four linked design pressures:
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pit depth
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retained airflow volume
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access for future maintenance
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structural and builder’s works coordination
Problem: The Rise Looks Small but the Service Constraint Is Big
A rise of around 357 mm does not sound like a major engineering obstacle on its own. In many buildings it would not be.
But here, the under-stair space was doing real work for the building. It was not empty. It was part of the extract arrangement. The challenge therefore was not “Can a lift do this rise?” but “Which lift geometry protects the air space best?”
That distinction matters. It is where many projects go wrong.
Solution: Design the Pit Around the Void, Not the Void Around the Pit
At Sesame, the design approach is to map the pit requirement against the service reality below.
That often means one of three approaches:
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using a split-depth geometry
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using the whole pit zone where the site allows it
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separating the step movement from the main platform movement so the occupied volume is reduced
This is why the Edinburgh Access Lift, Wellington Lift, and Waterloo Lift are useful reference points for early-stage thinking, even though each live project remains bespoke. The current product pages show that the Edinburgh uses a two- or three-tread arrangement with one large tread, the Wellington uses the bottom step as part of the lift table up to a 500 mm rise, and the Waterloo provides a rising platform with a much shallower minimum pit depth than the Wellington.
Which Sesame Lift Logic Best Suits an Air Extraction Void?
The answer depends on where the project can tolerate depth and where it cannot.
The live Edinburgh Access Lift page explains that this model is designed around two or three small treads followed by one large tread, and it is typically selected when the site has a short flight of steps and a generous upper landing. It is also described as shallower beneath the large landing than models such as the Wellington.
The live Wellington Lift page explains that the Wellington uses the bottom step as part of the lift table up to a rise of 500 mm, but it also requires a much deeper minimum pit below lower landing than the Waterloo.
The live Waterloo Lift page shows a minimum pit depth below lower landing of 442 mm plus cladding and bedding, compared with 1130 mm plus cladding and bedding for the Wellington.
For this kind of air extraction condition, that comparison is useful because it highlights the real engineering trade-off: deeper pit systems can be effective when the whole zone is available, but independent or shallower configurations become more attractive when preserved void volume matters.
Why the Independent Step Matters So Much
The key breakthrough in the transcript was moving away from the idea that the whole stair and lift arrangement had to behave as one deep, continuous mechanism.
Instead, the thinking shifted towards making the lower step independent from the lift platform.
“This step itself becomes independent, completely independent.”
Later in the conversation, the same principle was clarified again:
“One platform and one independent step that goes up and down.”
That is the important design lesson.
Once the step becomes its own moving element, the lift no longer needs to claim the same volume in the same way as a more integrated retracting stair arrangement. In air extraction situations, that can be the difference between a viable concept and a non-starter.
Volumetric Pit Modelling: The Part MEP Teams Actually Need
A project like this should not rely on a generic pit note or a vague “allow for lift” box in the model.
MEP teams need a proper understanding of:
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how much depth is required where
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how much volume is removed from the extract zone
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what remains open for airflow
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whether maintenance access is still possible
This is why volumetric pit modelling is such a useful concept-stage service from Sesame. It gives the building services team something practical to test instead of forcing them to guess. It also reduces the risk of the lift being blamed later for an airflow problem that was never properly modelled at the start.
This Approach Works When… / This Approach Is Unsuitable When…
| This approach works when… | This approach is unsuitable when… |
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| The stair rise is short but the under-stair zone is doing important service work | The team expects a standard off-the-shelf lift to be dropped in late without concept-stage coordination |
| The project can define exactly where depth is acceptable and where volume must be preserved | The extract strategy cannot tolerate any meaningful reduction in void volume and no rerouting or redesign is possible |
| The MEP team wants dimensional blocks or sectional information to test extract performance early | The site has no service access strategy for the remaining void or no way to maintain the extract zone |
| The architectural team is open to an independent step or split-depth arrangement | The project brief assumes a uniform pit everywhere regardless of the building services condition |
| Drainage and long-term pit water management are being coordinated at the same time | The pit is treated as “someone else’s problem” and drainage, sump position, and alarms are left unresolved |
Why Standard Calculations Often Fail
One of the most common mistakes is assuming that a small rise automatically means a simple lift.
It does not.
Another mistake is assuming that a lift pit is just a single depth across the whole footprint. In service-constrained entrances, that assumption can be completely wrong.
A third mistake is assuming the MEP system always has to move. Sometimes it does. But very often the better solution is to adapt the lift geometry first and only alter the services where absolutely necessary.
That is part of what makes this type of work specialist design rather than routine specification.
Common Misconceptions
Misconception: any standard platform lift can be adapted later once the MEP model is further along.
Correction: once the air extraction zone is structurally and environmentally committed, late lift selection becomes much harder. The lift usually needs to be considered early, with real geometry and real pit assumptions.
Misconception: if there is an extraction void, the services must always be rerouted.
Correction: not always. In many projects the smarter move is to reduce or reorganise the lift’s occupied volume instead.
Misconception: reducing pit depth always weakens the engineering solution.
Correction: not automatically. A shallower or split-depth design can be the correct engineering response if the mechanism layout and maintenance logic are handled properly.
How Drainage Fits Into This Type of Project
This article is mainly about air extraction, but drainage cannot be ignored.
Any lift pit or partially occupied void is still vulnerable to water ingress, standing water, and coordination failures between lift works and builder’s works. Sesame’s current Knowledge Hub guidance already explains that drainage should be planned as part of the lift strategy, not left as a generic afterthought, and that sump and conduit positioning must be coordinated early to avoid clashes and delays.
That is why this page should sit alongside:
Those pages deal more directly with pump logic, sump layout, alarms, and water management. This page deals with the additional complexity of preserving airflow and extract performance while still delivering an accessible entrance.
Product Integration Summary
| Sesame reference approach | Why it is relevant here | Key trade-off |
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| Edinburgh Access Lift | Useful when a project has a short stair run and a larger landing area, and where part of the landing zone can accept more depth than another part | The front section can still require deeper accommodation even if the large landing area is relatively shallow |
| Wellington Lift | Useful when the geometry suits a vertically retracting stair concept and the site can tolerate deeper pit logic | Greater overall pit demand can be harder to reconcile with a protected extract void |
| Waterloo Lift | Useful as a reference point for shallower platform logic and early conversations about reducing occupied pit depth | Independent platform logic can affect layout and user interface decisions, so the whole entrance needs to be considered together |
| Bespoke Lift | Often the right route when no single standard geometry cleanly solves the air extraction, stair, and landing relationship | Requires earlier concept input and more project-specific coordination |
Why This Knowledge Hub Page Is Different
Many accessibility articles talk about stairs, aesthetics, or planning. This one is specifically about how a lift interacts with an active air extraction void below the stairs.
That makes it different from Sesame’s more general pages about hidden lifts, retracting stair lifts, heritage access, pit depth, or broad coordination workflows. The central issue here is environmental coexistence: how to fit accessibility into a space that the building is already using to breathe.
That is a more specialist question, and it needs a more specialist answer.
Frequently Asked Questions
Can a platform lift be installed above an air extraction void?
Yes, but it usually needs bespoke engineering. The key issue is not just whether the lift fits. It is whether enough service volume remains for airflow, maintenance, and long-term performance.
Does a small rise mean the design will be simple?
No. A modest rise can still be complicated if the space below the stairs is part of the building’s extract or smoke control strategy.
What is the best Sesame reference product for this kind of problem?
That depends on the geometry. The Edinburgh Access Lift, Wellington Lift, and Waterloo Lift are useful reference points, but a bespoke configuration is often the right final answer.
Why is independent step design so useful above an extract void?
Because it can reduce the volume the lift needs to occupy and make it easier to preserve the service zone below.
Do I still need to think about pit drainage if the main issue is airflow?
Yes. Air extraction and drainage are different issues, but both affect long-term reliability. That is why lift pit sump pump best practice and accessibility lift pit drainage design should be considered alongside this type of entrance design.
Can MEP teams model this accurately at concept stage?
Yes, provided they are given real sectional logic, minimum dimensional envelopes, and volumetric pit information rather than a generic placeholder.
What should architects ask at the beginning of a project like this?
Ask what sits below the stair now, how much of that volume can be lost, whether maintenance access must remain, and whether the lift can be adapted before the services are moved.
Call to Action
If your project has stairs above a service void, smoke extract zone, or under-stair air plenum, the earlier Sesame is involved, the more options you are likely to keep.
Book a Teams Meeting with one of our Project Managers to review the rise, landing geometry, pit depth, and airflow constraints before the concept design gets locked in:
https://www.sesameaccess.com/book-a-meeting
