Somewhere between RIBA Stage 3 and contract award, every developer faces the same structural choice: should the mechanical, electrical and public health design sit inside the contractor’s own team, or with an independent consultancy appointed by the client? Both models are legitimate and widespread, and both deliver good buildings when matched to the right project. They also fail in predictable ways when mismatched. The useful question is not which model is better in the abstract, because neither is. It is which risks your particular scheme can absorb, and which it cannot.
Two models, one legal standard
Start with what the law says, because it says the same thing to both. Under the dutyholder regime introduced through the Building Safety Act 2022, anyone carrying out design work, whether employed by a contractor or a consultancy, must have the skills, knowledge, experience and behaviours the work demands, and must refuse work that is beyond their competence. The government’s guidance on design and building work applies those duties to every designer on every project, and requires clients to take reasonable steps to confirm the competence of whoever they appoint.
Both routes also design against the same technical baseline: the Approved Documents covering energy (Part L), overheating (Part O), ventilation (Part F) and the rest. The choice between them is therefore not about permission or capability in principle. It comes down to where the engineers sit, who pays them, and what they are optimising for.
What does a contractor’s in-house design team do well?
It closes the gap between a design decision and its construction cost faster than any other arrangement. An engineer who sits beside the estimator and the buyer designs what the supply chain can actually deliver, at prices the tender can carry, on the programme the site will run. On design-and-build contracts this integration also produces single-point responsibility: one organisation owns both the design intent and the installed outcome, which simplifies liability and removes a whole category of interface dispute.
For schemes where the engineering questions are substantially settled, perhaps a repeat typology with a proven servicing strategy, or a design developed to a mature Stage 4 before tender, that alignment is precisely what a client should want. In-house teams are at their best when the task is “build the approved scheme efficiently”.
One structural pattern is worth naming plainly, though. It is a feature of the model, not a fault of any firm. A design team whose employer carries the construction cost will, quite rationally, weigh capital cost heavily in every decision. When the performance questions are closed, that is alignment. While they remain open, with the energy strategy unfixed, the overheating position undemonstrated and plant-space assumptions yet to be stress-tested, the same alignment can quietly narrow the client’s options before the client has seen them.
When does a project need an independent MEP consultancy?
When the design must answer to the client’s brief and the building’s long-term performance rather than to a build programme, and above all when the scheme’s value hangs on engineering decisions made before any contractor is aboard. Around 40% of all the UK’s energy is used within buildings, as CIBSE notes, and the regulatory response (a tightening Part L, the Part O overheating regime) has pushed the decisive MEP choices earlier and earlier in the project.
An ambient-loop air source heat pump network of the kind AMF engineered at Cyprus Beckton, where 211 all-electric apartments are served from a shared low-temperature loop, is fixed in its essentials long before a subcontractor prices it: loop temperatures, plant space, riser strategy, the heat pump selection behind every apartment’s running costs. Decisions of that order sit most safely with a designer whose only customer is the client.
Independence has a second use: scrutiny. A client-side consultancy can review contractor design proposals, test value-engineering suggestions against the performance the planning consent depends on, and price what a substitution actually costs over a thirty-year life rather than a tender period.
Fairness cuts both ways. The consultancy model carries its own structural weakness: distance from the supply chain. An independent practice that never talks to installers produces elegant drawings that are expensive to build. The practical test of any consultancy is how much delivery sits in its track record. Have its engineers coordinated federated BIM models against structure and architecture? Has its Stage 4 information actually been built from, not merely tendered against?
How does the Building Safety Act change the calculation?
It raises the price of immature design whichever model you choose, most sharply on higher-risk buildings. For residential buildings of at least 18 metres or seven storeys, the Building Safety Regulator must approve the design before work starts, and will take up to 12 weeks to assess an application for a new building. After approval, changes run through a formal change control plan, and major changes halt the affected work until the regulator approves them.
Finishing the design on site was always a commercial gamble. On higher-risk buildings it is now a regulatory liability too, and both models must adapt. In-house teams bring buildability into the frozen design early, which the gateway regime rewards. Independent consultancies bring the documentation culture the regime demands: a complete, coordinated, evidenced design rather than a priced intent. AMF’s full Stage 4-7 appointment at Orion Park in West Ealing, where 95 all-electric apartments include a higher-risk building under the Act, is built around exactly that discipline: the design submitted for approval has to be the design that gets built.
A practical way to decide
Neither model needs defending; each needs matching.
- Contractor in-house design fits best when the employer’s requirements are genuinely fixed, the servicing strategy is proven, programme certainty outranks design optionality, and single-point design-and-build responsibility carries real commercial value.
- An independent consultancy fits best when the energy strategy is still live, the scheme is a higher-risk building, planning conditions rest on modelled performance, the client needs advice with no stake in the construction cost, or operating cost matters as much as capital cost.
- The hybrid is often strongest: an independent consultancy develops the design to a mature Stage 4, then is either novated to the contractor or, frequently better for the client, retained client-side to monitor delivery against intent while the contractor’s team details for construction.
Made deliberately at Stage 2 or 3, this choice costs nothing. Left to default at tender, it is often the most expensive unexamined decision on the project.
If you are weighing these models for a residential or mixed-use scheme, or deciding whether to novate or retain your consultant, AMF has worked on both sides of the arrangement: client-side Stage 4-7 appointments on one hand, MEPH drawing production and BIM coordination inside contractor-led teams on the other. Talk to us early, while the decision is still cheap.