A G99 DNO connection application that arrives without adequate structural documentation, or with a structural report that does not align with the rest of the application pack, will stall. The DNO will issue a Request for Information; the project programme will absorb the delay; and the installation team will spend time chasing a problem that was avoidable from the outset.
DNO application rejections and information requests relating to structural documentation are almost always avoidable. They arise from a small set of recurring errors that this article addresses directly.
What G99 Is and Why Structural Documentation Is Part of It
Engineering Recommendation G99 is the standard governing the connection of generating plant to a Distribution Network Operator's network at voltages above 1 kV. For commercial solar PV installations above 50 kW in Great Britain, a G99 application to the local DNO is the route to grid connection authorisation.
G99 applications are submitted to the DNO's connections team and reviewed by a network planner. The planner assesses whether the proposed generating plant will connect safely, operate within network technical limits, and has been adequately documented. Structural documentation forms part of the technical evidence pack because the DNO is confirming that the installation is technically sound, not just electrically, but as a complete physical installation on a building that is structurally capable of supporting it.
The structural section of a G99 application is not subject to a detailed engineering review by the DNO network planner. The DNO is not re-checking the structural calculations. They are confirming that a qualified structural engineer has reviewed the building and provided a written sign-off consistent with the application details.
What DNOs Look For in the Structural Evidence
A qualified signatory. The structural report or structural confirmation letter must be signed by a structural or civil engineer. Many DNOs are now explicitly requesting evidence of the signatory's professional qualifications. A report signed by an unqualified party will not pass DNO review.
Consistency with the application. The structural report must reference the same building, the same roof area, and the same array configuration as the G99 application. If the application specifies 220 panels at a defined layout on the south-facing roof of Building A, the structural report should confirm structural adequacy for 220 panels at that configuration on that roof. A structural report that was produced at an earlier design stage with a different panel count, or that references the building generally without identifying the specific roof area, creates an inconsistency that the DNO will flag.
PI insurance confirmation. Some DNOs specifically request confirmation of the structural engineer's professional indemnity insurance as part of the G99 structural evidence pack. Where this is not included, an information request will follow. Including PI insurance confirmation in all structural report submissions avoids this delay.
MCS compliance reference. For MCS-certified installations, the structural documentation should reference MCS MIS 3002 Section 5.9 compliance. Some DNO network planners specifically check for this reference to confirm the structural sign-off format is consistent with the MCS certification standard that will apply to the installation.
The MCS MIS 3002 and G99 Relationship
MCS MIS 3002 Section 5.9 and the G99 application process are parallel requirements. They run simultaneously and both require structural engineer documentation, but they serve different functions:
MCS MIS 3002 Section 5.9 is the certification body's requirement: it confirms the installation meets the MCS technical standard, which is a condition of the MCS certificate being issued and of any associated financial incentive (REGO, smart export guarantee) being available.
G99 is the DNO's requirement: it confirms the installation is technically sound for connection to the distribution network.
A single well-structured structural report addresses both requirements simultaneously. There is no engineering reason to produce separate structural documentation for MCS purposes and G99 purposes. One report, correctly formatted, signed by a qualified structural engineer, and referencing the applicable standards, satisfies both. Projects that produce separate documentation for each requirement are creating unnecessary duplication and increasing the risk of inconsistency between the two document sets.
Common Errors That Delay G99 Applications
Array configuration mismatch. The most common cause of structural-related G99 delays. The structural report was produced at an early design stage. The array configuration changed between the report date and the G99 submission date. The application references the revised configuration; the structural report references the original. The DNO identifies the discrepancy and issues an RFI.
Resolution: ensure the structural report is updated to reflect the final array configuration before G99 submission. At Solar Surveys, supplementary calculation notes confirming revised configurations are produced quickly where the structural adequacy finding is not affected by the change.
No qualified signatory. The structural report is signed by an energy assessor, a roof surveyor, or a person without structural engineering membership. The DNO flags this and requests replacement documentation from a qualified structural engineer. The project is delayed by the time required to commission and produce a compliant report.
Generic building description. The structural report covers “the building generally” without identifying the specific roof area and array layout. For a multi-building site or a building with multiple roof elevations, this creates ambiguity that the DNO will not resolve in the applicant's favour.
No reference to site wind speed. Some structural reports confirm structural adequacy without referencing the site-specific wind speed used in the uplift calculation. A DNO network planner reviewing a site on the Scottish coast may reasonably question whether the wind uplift calculation was appropriate for the exposure conditions, if the site wind speed is not stated.
Turnaround Impact on G99 Programme
G99 applications involve a defined review period of 65 business days for standard applications under the current ENA framework. The clock starts from the formal submission date. Where structural documentation is missing from the initial submission, and is submitted after the formal application has been accepted, the DNO may treat the complete structural evidence as a material amendment, potentially resetting the review period.
Ensuring all structural documentation is in place before G99 submission avoids this scenario. Solar Surveys' 48-hour desktop report benchmark means that G99-ready structural documentation can be in place within 48 hours of instruction confirmation. For project teams working to G99 submission programme dates, this turnaround allows the structural evidence to be confirmed before the submission window, not scrambled together during it.
A G99 application delayed by missing or non-compliant structural documentation is a programme failure that was avoidable. The structural report should be the first item confirmed after array design is finalised, not the last.
Engineering Recommendation G99: Structural Evidence Requirements
Engineering Recommendation G99 governs the connection of generating plant to the distribution network in Great Britain for installations exporting above 16A per phase at low voltage. While G99 is primarily an electrical engineering recommendation, it references structural requirements in the context of generating plant installation compliance and building safety, and DNOs processing G99 applications may request structural evidence as part of their technical review of the application.
The most direct G99-related structural requirement arises where the installation modifies the building in a way that requires building regulations approval. For G99-connected solar installations that exceed Permitted Development thresholds, or that involve structural modifications such as reinforced fixings or structural frame modifications, Building Regulations compliance confirmation may be required as part of the planning and consent documentation that the DNO technical reviewer assesses alongside the G99 application. The structural engineer’s signed report, confirming that the installation meets the required structural standards, is a component of this Building Regulations evidence package.
DNOs also occasionally request structural evidence directly in the context of assessing whether the installation location, particularly the location of inverters, isolation switches, and metering equipment, meets the access and safety requirements in G99. Where equipment is proposed to be mounted on the building fabric in locations that require structural fixings beyond standard light fittings, heavy wall-mounted inverter cabinets, cable management systems, or external equipment pods, the structural engineer may be asked to confirm that the building fabric at the proposed equipment location can support the loads.
Multi-Phase and Phased G99 Applications: Structural Coordination
Large commercial solar installations are sometimes developed in phases, an initial phase commissioned and grid-connected, with subsequent phases added as budget, planning consent, or grid capacity becomes available. Multi-phase G99 applications present a structural coordination challenge: the initial phase is structurally assessed for clearance, but the subsequent phases may involve additional loading on a structure that has already been assessed and is already carrying the Phase 1 array. The structural clearance for each phase must be confirmed cumulatively, not independently.
The cumulative structural assessment issue arises most clearly on buildings where the initial phase has consumed a significant portion of the available residual structural capacity. If Phase 1 clearance is issued with a maximum dead load constraint that uses the full remaining capacity, a Phase 2 addition cannot be structurally cleared without structural upgrade or without reducing the Phase 1 loading to create headroom for Phase 2. The structural engineer handling Phase 2 must be provided with the Phase 1 structural report and the as-installed Phase 1 specification to confirm the actual loading already applied before assessing the additional Phase 2 load.
For the DNO, a phased installation creates G99 application sequencing requirements. Each phase may be assessed under a separate G99 application, or the full development may be assessed under a single G99 application that anticipates the eventual total export capacity of all phases. The structural assessment strategy should be coordinated with the G99 application strategy from the outset, if the DNO requires a G99 application for each phase, the structural evidence should be provided phase by phase; if the DNO is assessing the full development under a single application, the structural evidence should address the total cumulative loading of all phases.
Grid Connection Delay and Structural Report Validity
G99 application timescales are a known source of programme uncertainty for commercial solar projects. DNO processing times for complex G99 applications can extend to several months, and in some network areas the queue of pending applications has extended this further. Projects that obtain structural clearance early in the development programme, for the efficient reasons discussed throughout this article, may find that their structural report is approaching its conventional validity window by the time the G99 application is resolved and installation can begin.
Where a material delay between structural clearance and installation start is anticipated, the structural engineer should be informed so they can advise on whether the report remains valid and whether any re-inspection or re-confirmation is appropriate. For buildings where the clearance was unconditional and the condition survey found no structural deterioration concerns, a report issued 18-24 months before installation start is generally still valid provided the building has not been modified and no significant structural events (storm damage, collision, adjacent excavation) have occurred in the interval. The structural engineer can issue a confirmation of continuing validity note at low or no cost, updating the report date and confirming that no material changes have occurred since the original assessment. This confirmation note satisfies lender and MCS requirements for report currency without requiring a full re-assessment.
For buildings where the original clearance was conditional or where deterioration was noted in the original survey, a longer installation start delay warrants a brief follow-up inspection to confirm that the condition noted at the original survey has not worsened. This is particularly relevant for buildings in coastal or industrial environments where the rate of structural deterioration is higher than for inland urban buildings, and where a 12-18-month delay could represent material progression of corrosion at critical structural connections. The cost of a brief follow-up inspection is small relative to the risk of proceeding on an installation programme without confirming that the structural basis for clearance remains valid.
G99 structural report submissions are assessed by DNO technical reviewers against three criteria: engineer qualification (the signing engineer must have demonstrable structural engineering competence); calculation completeness (the report must address dead load, wind uplift, and Eurocode load combinations for the specific building); and installation specificity (the report must reference the actual system being installed, not a generic array). A report satisfying all three criteria at first submission avoids the supplementary query round that adds weeks to G99 application timelines. Solar Surveys desktop structural reports are formatted to satisfy these three criteria from first issue.
WHERE SOLAR SURVEYS ADDS VALUE
G99-READY STRUCTURAL DOCUMENTATION: FIRST SUBMISSION
Structural reports from Solar Surveys are formatted to satisfy DNO G99 application documentation requirements from the outset. The report explicitly addresses structural adequacy under the dead load of the proposed array and mounting system, wind uplift resistance at the proposed fixing locations, and MCS MIS 3002 Section 5.9 compliance, the three elements DNO technical reviewers check before accepting a G99 application as complete. The 48-hour turnaround means structural documentation is available before the G99 application window opens, not after it.
CLIENT PROFILE
An EPC firm had experienced two G99 application returns from a DNO within a 12-month period, both due to incomplete or non-compliant structural documentation from their previous supplier. Switching to a standardised Solar Surveys report format, with explicit MCS MIS 3002 references, BRE Digest 489 wind uplift methodology, and a clear structural verdict section, resulted in no further application returns on subsequent submissions.
G99 Commissioning Tests and Structural Implications
The G99 commissioning test programme, conducted before the DNO issues final approval for energisation, includes functional tests of protection relays, power quality checks, and confirmation that the system operates within the agreed parameters. While these are primarily electrical tests, they have structural implications: the commissioning test programme requires personnel to access the rooftop installation, and the structural assessment sign-off should have confirmed that maintenance access loading was within structural capacity before this access occurs.
For large commercial installations where commissioning requires working at height on the roof surface, adjusting DC string connections, accessing monitoring equipment, or servicing roof-mounted inverters, the structural assessment should have included a maintenance access load check, confirming that the roof structure can carry the weight of maintenance personnel plus any equipment they bring onto the roof.
Annual Review and G99 Compliance
G99 connection agreements include ongoing obligations that the connected generation system continues to comply with the technical requirements of the Engineering Recommendation. These requirements include protection relay settings, power factor compliance, and generation limitation during specific grid conditions. Structural failure of any component of the rooftop installation, a fixing failure that allows panel movement, a racking system failure that affects panel positioning, could theoretically affect the electrical performance characteristics that the G99 approval was based on.
The structural inspection programme should therefore be co-ordinated with the annual G99 compliance review. Any structural anomaly found during inspection should be assessed for its electrical implications as well as its structural implications, and the DNO notified if the electrical performance of the system may have been affected.
Changes to Installed Systems and G99 Obligations
The G99 connection approval covers the installation as specified in the application. Material changes to the installation, additional panels, different inverter models, changes to the connection arrangement, require notification to the DNO and may require a new or amended G99 application. Structural changes that affect the electrical system (replacing panels with a different model that has different power output characteristics, for example) have both structural and G99 implications.
Asset managers planning modifications to commercial solar installations should obtain both structural engineering review and DNO consultation before proceeding. The structural engineer and the DNO are independent sign-off parties; approval from one does not substitute for approval from the other.
G99 for BESS Co-Located with Solar
Battery storage co-located with rooftop solar operates under the same G99 framework as the solar generation component, but introduces additional technical requirements, particularly for protection relay settings that account for both generation (solar export) and consumption (battery charging) operating modes. The G99 application for a combined solar-plus-storage system requires the DNO to approve a more complex protection scheme than for solar alone.
The structural implications of BESS co-location, additional cabinet point loads, fire suppression structural loads, must be cleared before the G99 application can include the BESS element. Where the structural assessment concludes that the rooftop cannot carry the proposed BESS cabinets, and the cabinets must be relocated to ground level, the G99 application must be revised to reflect the changed installation geometry. This iteration between structural assessment and G99 application is a programme risk that should be managed by running structural and grid connection workstreams in parallel from the outset.
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