Amy Norman, senior civil engineer, Charles Rothnie, senior structural engineer, and Ben Gillham, graduate structural engineer, from civil and structural engineering consultancy Perega, discuss the challenges that the 10 per cent Biodiversity Net Gain (BNG) mandate has had when it comes to operationalising ecological thinking within the constraints of developing existing healthcare estates.
The 10 per cent Biodiversity Net Gain (BNG) mandate came into force in February 2024,1 marking a fundamental shift in how healthcare infrastructure projects must be designed and delivered. For NHS trusts managing complex, live estates, we are now firmly in the implementation phase. The question is no longer whether to integrate ecological uplift, but how to operationalise it within the physical and budgetary constraints of existing hospital sites.
This transition presents what we might call the ‘Brownfield Paradox’. The very structural interventions required to deliver nature-based solutions on constrained urban sites can risk increasing a project’s embodied carbon footprint. Green roofs require stronger frames. Sustainable Drainage System (SuDS) features demand careful ground engineering. Biodiversity zones require access routes for maintenance. Every ecological enhancement can create a cascade of structural implications that must be balanced against the carbon cost of implementing and maintaining the supporting infrastructure.
The solution lies in treating BNG as a strategic engineering consideration from day one of the master planning period, rather than a regulatory box-ticking exercise bolted on during the detailed design stage.
The challenge of restricted footprints
Delivering a 10 per cent biodiversity gain on a busy hospital site presents fundamentally different challenges to more typical greenfield developments. Space is at a premium. Usually, every square metre of ground-level estate is already committed to circulation, servicing, emergency access or temporary clinical accommodation. This spatial constraint forces estate managers to look upwards and outwards, exploring opportunities in vertical spaces and external works.
Rooftop solutions appear attractive in principle. However, the reality is more complex. Essential plant rooms, lift overruns, emergency helicopter access, and maintenance zones all compete for roof space. This competition for rooftop real estate makes the integration of green or brown roofs a complex planning exercise requiring careful prioritisation of competing operational demands.
When green or brown roofs are technically feasible, they introduce significant dead loads that the existing structural frame may not have been designed to accommodate. There is a considerable payback time for a green roof’s production and maintenance, and the economic case must also account for the increased material usage in the supporting structure: thicker slabs, larger columns, additional reinforcement. The carbon penalty can be substantial.
This is where strategic load path management becomes essential. Rather than treating every roof area as equally viable, engineers must identify zones where the existing structural grid offers inherent capacity. Locating biodiversity features over shorter structural bays or areas with existing stiffness minimises the need for high-embodied-carbon strengthening works.
The technical nuances of external works
External works represent some of the most cost-effective opportunities for BNG delivery on healthcare sites, but they demand careful technical consideration at the master-planning stage.
Sustainable drainage systems offer perhaps the clearest win. Swales, detention ponds, and rain gardens can be cost-neutral or even cost-negative compared to traditional buried tank solutions, while delivering measurable ecological benefits. The key is early integration into the site layout. A swale designed into the landscape from the outset becomes an attractive, biodiverse feature. The same intervention retrofitted into a constrained site becomes an expensive engineering challenge.
Understanding site hydrology is crucial. Surface water flow paths and infiltration characteristics need to be mapped before the building footprint is fixed. This allows SuDS features to be positioned where they can function naturally, using hydrophytic planting to support aquatic life and enhance local ecology without requiring complex pumping or attenuation infrastructure.
Foundation design also plays a critical role in preserving the existing ecological baseline. Root Protection Areas around mature trees can represent significant biodiversity units. Where site constraints demand building near established vegetation, smaller diameter piles, or cantilevered ground beams can be designed to allow for root zones without compromising tree health. This sensitive approach preserves existing units, making the 10 per cent biodiversity net gain calculation considerably more achievable.
The Midland Metropolitan University Hospital2 serves as a benchmark for integrated external works. In this instance, the drainage strategy was connected directly with the local canal network. This approach transformed a regulatory requirement into a genuine landscape asset, creating wetland habitats while managing flood risk and reducing reliance on grey infrastructure.
Navigating clinical and operational priorities
For NHS estate managers, BNG is one consideration among many competing demands. Clinical safety, infection control, operational resilience, and whole-life costs must all be balanced alongside ecological targets. The challenge is finding solutions that serve multiple objectives simultaneously.
This is where the timing of engineering involvement becomes critical. When structural and civil engineers are brought in after the strategic brief has been finalised, options become limited. BNG features are squeezed into whatever space remains. By contrast, early engagement allows the building’s structural DNA and surrounding site to be shaped around integrated ecological solutions.
Phased development programmes offer opportunities to protect biodiversity during construction. Careful sequencing of works can avoid vegetation clearance during bird nesting seasons, helping to maintain ecological corridors through the site and establish new planting ahead of demolition. These measures require minimal additional cost when planned from the outset, but become prohibitively complex when retrofitted into a locked-down programme.
The adaptive reuse of existing structures represents perhaps the most efficient route to meeting BNG requirements while managing carbon and cost. The NHS Dorset Health Village demonstrates this approach at scale.3 A former department store was transformed into a modern primary care facility, with 70 per cent of decommissioned materials and equipment from a former Nightingale Hospital repurposed on site. This circular economy approach avoided the carbon and ecological cost of new-build construction, freeing up capacity to invest in meaningful ecological enhancements to the surrounding landscape.
The clinical case for green infrastructure
Beyond regulatory compliance, there is a growing evidence base linking biodiversity integration to improved patient outcomes. Several studies suggest a positive correlation between access to natural light and green spaces and improved patient recovery, including reduced hospital stays and decreased analgesic requirements.4 This evidence builds on pioneering research showing that patients with views of nature recover faster post-surgery.5 For estate managers facing pressure to improve patient experience and staff wellbeing, ecological design offers tangible clinical benefits.
Maggie’s Leeds Centre6 provides a compelling exemplar. The facility’s accessible green roof functions as a therapeutic sanctuary, with 17,000 plants that patients are encouraged to care for as part of their recovery. The structural design carefully positioned the roof garden to maximise natural light penetration while managing dead loads through strategic placement over zones with inherent structural capacity.
For acute hospital settings, where outdoor access may be limited by operational constraints, carefully positioned courtyards, healing gardens, and views onto biodiverse landscapes can deliver similar benefits. The engineering challenge is creating these spaces without compromising clinical adjacencies or operational flow.
Redefining value engineering
The traditional approach to value engineering viewed green features as discretionary costs to be stripped out when budgets tighten. With BNG now a statutory requirement, this mindset must shift. The question is no longer whether to deliver ecological uplift, but how to do so most efficiently.
This requires engineers to move beyond the ‘bolt-on’ sustainability approach. Instead of adding green roofs and SuDS features to a conventionally designed building, we must embed ecological thinking into the fundamental civil and structural design. Integrating the structural grid with landscape vision from the outset creates multi-functional elements that minimise material waste. Early site appraisal, working closely with arboriculture experts, can inform building placement and foundation strategies that protect existing biodiversity units.
Value engineering must now focus on optimising the delivery of green features, not questioning whether they are required. This might involve measures such as relocating a detention pond to avoid contaminated ground, selecting plant species that sequester carbon while supporting local ecology, or using green roof build-up as part of the thermal performance strategy to reduce mechanical plant loads.
A call for early integration
For NHS trust leaders and estate managers, the clear recommendation is to involve civil and structural engineers at the earliest possible stage of master-planning. BNG should be treated as a baseline design requirement, not a late-stage problem to be solved with off-site credits or biodiversity banking.
The 10 per cent mandate represents an opportunity to create healthcare estates that deliver genuine ecological value while enhancing clinical outcomes and operational resilience. However, realising this potential requires moving beyond compliance checklists to genuine integration of engineering and ecological design.
The engineering toolkit is proven: sensitive foundation design protects existing trees, SuDS delivers cost-neutral drainage solutions, strategic load path management accommodates green roofs without carbon penalties, and circular economy principles unlock the value in existing assets. With these methods now being employed across healthcare projects, what is required is the commitment to embed these approaches from the strategic brief stage; ensuring that the 10 per cent Biodiversity Net Gain is achieved through smart civil and structural design, not costly last-minute interventions.
The implementation phase of the BNG mandate will test whether the healthcare sector can operationalise ecological thinking within the constraints of existing estates. With early engineering engagement and strategic integration, NHS trusts can meet this regulatory requirement while building more resilient, patient-centred and sustainable healthcare environments.
Amy Norman
Amy is a senior civil engineer at Perega with extensive experience in civil design and statutory adoptions. She focuses on the early integration of sustainable drainage systems (SuDS) to enhance site biodiversity. Amy’s expertise in drainage design is vital for delivering cost effective, nature-based solutions within sites.
References
1 https://www.gov.uk/guidance/understanding-biodiversity-net-gain
2 https://worldlandscapearchitect.com/nature-inspired-healing-at-midland-metropolitan-university-hospital/?v=b870c45f9584
3 https://www.nationalhealthexecutive.com/articles/video/university-hospitals-dorset-become-first-nhs-trust-launch-think-big-project
4 https://www.commonwealthfund.org/publications/newsletter-article/focus-health-care-leaders-seek-create-healing-environments-patients
5 https://pubmed.ncbi.nlm.nih.gov/6143402/
6 https://www.archdaily.com/941540/maggies-leeds-centre-heatherwick-studio