Solar Feasibility Assessment for Commercial Property in the UK: What It Covers and What Comes Next

A solar feasibility assessment tells you whether a commercial roof can support a photovoltaic system, technically, structurally, and commercially, before you commit capital or design resource. Done properly, it prevents two expensive failure modes: investing in detailed engineering on a roof that will never pass structural sign-off, and abandoning a project mid-procurement because a loading problem emerges late.

This article explains what a rigorous solar feasibility assessment covers for UK commercial property, which professional inputs are required at each stage, and how to sequence the process so that structural clearance is never the last thing you discover.

What a Solar Feasibility Assessment Actually Covers

The term is used loosely across the industry. Roofing contractors sometimes call a site visit with a rough layout sketch a "feasibility." For commercial properties, a credible feasibility assessment has three distinct technical workstreams that must run concurrently.

All three workstreams can produce a "no" independently. A structurally marginal roof with ideal southern orientation and a strong grid connection is still not viable if the structure cannot be reinforced cost-effectively. The feasibility stage exists to find these blockers before procurement.

The Structural Workstream in Detail

For most commercial properties, particularly those built before 2000, the structural assessment is the highest-risk workstream. Original design drawings may be missing, the roof may have been re-covered, and as-built loading capacity may be unknown.

The structural workstream at feasibility stage typically involves three activities:

Drawing review: A structural engineer reviews available drawings to identify the primary frame type, purlin section and span, connection details, and any previous modifications. This takes half a day for a straightforward shed and several days for a complex multi-span facility.

Preliminary loading assessment: Using the array layout from the energy modeller, the structural engineer calculates the incremental dead load (typically 10-18 kg/m² for a standard rooftop array) and checks it against available capacity. If spare capacity exists without modification, the feasibility is structurally viable. If not, the engineer scopes what reinforcement would look like, and at what cost.

Intrusive investigation (if required): Where drawings are absent or structural members are non-standard, the engineer may specify an intrusive survey: opening up ceiling voids, taking section measurements, or extracting material samples for testing. This converts a conditional "possibly viable" to a definite answer.

Grid Connection: Why DNO Pre-Applications Are Non-Negotiable

One of the most common commercial solar project failures occurs when an installer completes design and procurement and then submits a G99 application, only to discover the local network cannot accept the proposed export. DNO capacity constraints are geography-specific, not predictable from array size alone, and can change between applications.

A credible feasibility assessment includes a formal DNO pre-application, not an installer's opinion on likely capacity. Pre-applications are free in most cases, take four to eight weeks to receive a formal response, and provide a network capacity figure that can anchor the commercial case. Export limiting, import-only batteries, or full grid reinforcement costs become quantifiable at this stage rather than at financial close.

Roof Condition and its Interaction with Structural Viability

A structurally sound frame can still produce a non-viable feasibility if the roof covering is near end-of-life. Replacing a metal deck or membrane after PV installation is significantly more expensive than replacing it before, the array must be demounted, re-roofing completed, and the array reinstated. For a 500 kWp array, this can represent £80,000, £150,000 of avoidable cost.

Feasibility assessments for properties with roof coverings over fifteen years old should include a thermal imaging drone survey to detect delamination, trapped moisture, and membrane failures. A roof in poor condition does not necessarily kill the project; it changes the capital expenditure sequencing, re-roofing becomes part of the project cost, not an excluded item.

Planning and Heritage Constraints

Commercial rooftop solar generally benefits from Permitted Development Rights under Class J of the Town and Country Planning (General Permitted Development) Order 2015, subject to conditions on proximity to designated land and array height above roof plane. However, some commercial properties fall outside permitted development:

• Listed buildings require Listed Building Consent and, typically, planning permission
• Properties in Conservation Areas may have Article 4 directions removing permitted development rights
• Buildings within the curtilage of scheduled monuments require Scheduled Monument Consent
• Some industrial sites in enterprise zones have site-specific conditions attached to their original planning permission

A feasibility assessment should include a planning desktop check, a one-day exercise that identifies any constraints before design resource is committed. The structural engineer and planning consultant can work in parallel; there is no need to sequence these.

How to Structure the Feasibility Timeline

The most efficient feasibility process runs all three technical workstreams concurrently from day one, with a single gate meeting at week six to eight where all outputs are reviewed together. Sequencing them, structural first, then grid, then energy, doubles the timeline and creates dependencies that can delay the project by months.

What the Feasibility Output Should Contain

A robust feasibility report for a commercial property should provide enough information to make a go/no-go decision and, if proceeding, to scope the detailed design phase. It should contain:

• Structural assessment: current loading capacity, proposed array load, viability verdict, any reinforcement scope and indicative cost
• Roof condition: current state, estimated remaining life, any pre-PV works recommended
• Grid connection: DNO pre-application reference number, available export capacity, any constraint conditions
• Energy model: indicative annual generation (kWh), self-consumption forecast, export forecast, and payback at assumed electricity prices
• Planning: permitted development eligibility confirmed or planning application required with indicative cost and timeline
• Programme: indicative design, procurement, and construction timeline to energisation

Cost Benchmarks for Feasibility Work

Feasibility costs vary with building complexity, available documentation, and whether intrusive investigation is required. For the structural assessment component of a feasibility study, desktop structural reports are priced on application. For complex or multi-span buildings requiring on-site assessment, contact Solar Surveys for a fee proposal.

These figures represent a small fraction of project value. A 500 kWp rooftop solar project carries a construction cost of £350,000, £500,000. Identifying a structural or grid constraint at feasibility, rather than at financial close or during installation, typically avoids cost overruns an order of magnitude larger than the feasibility fee.

Common Feasibility Mistakes

Using installer-provided structural opinions: Solar installers are not structural engineers. An installer who says "the roof looks fine" has not conducted a structural assessment. MCS certification requires sign-off from a structural engineer, and the earlier in the process that person is involved, the cheaper the project will be.

Assuming permitted development applies: Class J permitted development has specific conditions. A brief planning check at feasibility costs far less than an enforcement notice or retrospective consent application.

Omitting a DNO pre-application: Grid connection assumptions made without a formal DNO response are commercial fiction. They produce business cases that may be fundamentally unviable.

Treating feasibility as an internal desk exercise: Property and asset management teams without technical backgrounds are not qualified to conduct solar feasibility assessments. The technical workstreams require structural engineers, energy consultants, and planning professionals working from site data, not assumptions.

When Feasibility Becomes a Structural Survey

Once a feasibility concludes "proceed," the next step is a full desktop structural report or, where drawings are absent or the structure is non-standard, a combined desktop-and-site structural survey. This is the document that certifies the roof as capable of supporting the as-designed array and is required for MCS certification and lender due diligence.

The feasibility structural workstream and the detailed survey are related but distinct. Feasibility answers "is it worth proceeding?" The survey answers "what exactly can this roof carry and what exactly does it need?" Procurement and lender sign-off require the survey, not the feasibility.

For commercial property owners and asset managers, the most efficient approach is to retain the same structural engineer from feasibility through to survey sign-off. The knowledge built during feasibility, understanding the frame, reviewing the drawings, identifying any anomalies, makes the detailed survey faster and cheaper than starting fresh with a new firm.

Grid Capacity and Structural Viability: How the Two Assessments Interact

A commercial solar feasibility assessment addresses two independent constraints that both must be satisfied before a project is viable: grid connection capacity and structural capacity. Developers who assess only one of these constraints in the early stages of project development frequently encounter late-stage programme delays when the unconsidered constraint surfaces. The most efficient feasibility process addresses both in parallel, from the earliest point at which basic project parameters are defined.

Grid connection feasibility is primarily determined by the available export capacity at the nearest appropriate connection point on the distribution network operator’s network, and the cost and timescale of providing or upgrading that connection. For commercial rooftop solar, connection at low voltage (LV, 230/400V) is appropriate for installations up to approximately 100 kWp in many network areas. Above this scale, a high voltage (HV) connection at 11 kV is typically required, involving a more complex application process and longer connection timescales. The DNO’s feasibility response, which indicates available capacity and indicative connection cost, can be obtained within a few weeks of submitting a connection enquiry, making it one of the faster early-stage feasibility inputs to obtain.

Structural feasibility, by contrast, is primarily determined by the physical condition and load-carrying capacity of the specific building, which must be assessed by a structural engineer. Desktop structural assessment can be completed quickly, typically within 48 hours of a complete instruction, and the cost is low relative to the project development costs it unlocks. Running structural and grid feasibility in parallel, from the point at which a specific building and array specification have been identified, reduces the total feasibility timeline compared to running them sequentially.

The interaction between grid and structural assessments is most significant when grid connection constraints force a change to the array specification. If the DNO’s connection assessment limits the export capacity to 50 kWp, lower than the originally proposed 150 kWp, the array may need to be redesigned at a smaller scale, with different panel density, racking layout, or roof coverage. A structural assessment conducted against the original 150 kWp specification must be reissued or confirmed as valid for the revised 50 kWp specification, typically straightforward if the revision reduces array density, as the structural load is reduced. This interaction is manageable if both assessments are tracked jointly through the feasibility process; it creates rework and delay if they are managed separately.

Financial Viability: What Structural Assessment Contributes to the Business Case

Structural assessment outputs are not just a regulatory compliance requirement, they directly contribute to the financial modelling of a commercial solar project. The structural clearance verdict and its conditions determine the maximum viable array capacity on the building, which is the primary input to energy yield forecasting and therefore revenue modelling.

The maximum permissible array dead load stated in a conditional structural clearance directly constrains the panel specification available for the project. A heavier bifacial panel with a higher wattage output may generate more revenue per panel than a lighter standard panel, but if the heavier panel exceeds the dead load limit in the structural clearance, it cannot be used without a supplementary structural assessment. The financial model must reflect the actual panel specification that can be installed within the structural envelope, not the specification that would be optimal from an energy yield perspective alone.

The wind uplift fixing requirements stated in the structural report translate into installation labour cost. Edge zone enhancements, closer fixing centres, additional racking ties, require more installation time per unit area in the affected zones. For large roofs where edge zones constitute a significant proportion of total array area, this fixing enhancement cost can be material to the project economics. Financial models that use a uniform installation cost per panel across the full array area without accounting for edge zone fixing enhancements will underestimate installation cost on buildings with demanding wind load conditions.

Structural remediation costs, where the structural assessment identifies physical work required before installation can proceed, must be incorporated into the project budget at the feasibility stage if the desktop assessment identifies remediation risk. On older buildings, the probability of encountering some structural remediation requirement is non-trivial, and feasibility budgets that make no provision for remediation risk are routinely revised upward when the structural assessment is conducted. Including a contingency allowance for structural remediation in feasibility budgets, typically 2-5% of construction cost on pre-2000 buildings, is a standard risk management practice that reflects the actual distribution of outcomes in commercial portfolio development.

Solar Feasibility for Lease and PPA Structures: Specific Considerations

Commercial solar feasibility assessments conducted in the context of a roof lease or PPA transaction have additional dimensions that pure owner-occupier assessments do not. The building owner, the solar developer, and the energy offtaker each have distinct interests in the feasibility outcome, and the assessment must address the concerns of all three parties to be effective.

The building owner’s primary feasibility concern is structural: will the installation affect the integrity of the building, and is the structural clearance documentation adequate for building insurance and any landlord consent requirements? The structural report addresses both of these concerns directly. If the landlord has a preferred structural engineering firm specified in their standard licence to alter conditions, the assessment should be conducted by or reviewed by that firm before the licence application is submitted.

The solar developer’s primary concerns are financial viability and programme: is the site economically viable at the proposed tariff or contract price, and can structural clearance and regulatory approvals be obtained within the development programme? Desktop structural assessment addresses the programme element by confirming whether the structural workstream is on the critical path, and whether any on-site investigation requirement will extend the programme beyond standard timescales.

The offtaker’s concerns are output certainty and contractual risk: will the system deliver the contracted energy volume, and is there structural risk that could affect system availability? A clean structural clearance with no unresolved conditions reduces the offtaker’s technical due diligence concerns and supports a straightforward PPA or licence negotiation. Structural conditions that are not properly resolved, or that are inadequately documented in the project file, create uncertainty that sophisticated offtakers will identify and may use to negotiate reduced price or additional contractual protections.

WHERE SOLAR SURVEYS ADDS VALUE