The realistic 12-month picture
A typical 200-500 kWp UK office solar project takes 9-12 months from initial enquiry to commissioning. The steps and dependencies are well-established; understanding them helps project sponsors plan capex cycles and tenant communications.
Month-by-month breakdown
Month 1: Feasibility and proposal
- Half-hourly meter data request to DNO/supplier (typically 5-10 working days)
- Site visit and structural / roof assessment
- PVSyst yield modelling
- Planning route assessment
- Fixed-price proposal with all four funding routes
Customer decisions: funding route selection, scope decisions (battery yes/no, EV charging yes/no).
Month 2: Contract and design
- Detailed design phase
- Structural engineering sign-off
- Electrical design (string layout, inverter selection, cabling routes, switchroom modifications)
- Project programme finalisation
Customer decisions: install timing preferences (out-of-hours, weekend, summer-only), tenant communication strategy.
Month 3-4: Planning and DNO application
- Planning route execution: Permitted Development (no application), Prior Approval (56 days), full planning (8-13 weeks), Listed Building Consent (8-13 weeks)
- G99 grid connection application (typically 8-16 weeks DNO response time)
- Pre-application consultations with conservation officers (heritage settings)
Customer involvement: Minimal — we manage applications.
Month 5-6: Approvals and procurement
- Planning / Prior Approval determination
- DNO G99 acceptance
- Equipment procurement (panels, inverters, mounting, switchgear) — typical 8-12 week lead time on Tier-1 manufacturer products
- Site mobilisation planning
Customer involvement: Tenant communication if relevant.
Month 7-9: Installation
- Site mobilisation (typically 1 week)
- Mounting system installation (2-3 weeks for 300 kWp typical)
- Panel installation (1-2 weeks)
- Electrical installation (2-3 weeks)
- Commissioning testing (1 week)
Customer involvement: Site access coordination, parking management.
Month 10: Commissioning and grid connection
- Final inverter commissioning to IEC 62446
- DNO witnessing visit and connection approval
- Generation start
- Performance monitoring setup
- Customer training session
Customer involvement: Half-day commissioning attendance recommended.
Month 11-12: Handover and documentation
- 30-day post-commissioning performance review
- Documentation handover (electrical certs, structural certs, warranty certs, monitoring access)
- Scope 2 Disclosure Pack delivered
- SEG tariff registration
- O&M contract activation
Customer involvement: Document storage and tenant onward communication.
What drives schedule
Three factors materially affect timeline:
1. Planning route. Permitted Development can shave 3 months; full planning permission in heritage settings can add 3-6 months versus baseline.
2. DNO grid connection. In constrained networks (London UKPN areas, Greater Manchester ENWL, Bristol WPD), G99 acceptance can take 12-18 months. Always file G99 early — it runs in parallel with planning, so doesn’t directly add to programme.
3. Customer decision speed. From proposal to contract signing, customers typically take 4-12 weeks. Faster customers (sub-4 weeks) compress overall timeline; slower customers (12+ weeks) extend it linearly.
When the timeline compresses
The fastest UK office solar project we’ve delivered ran feasibility to commissioning in 5.5 months. The conditions:
- Permitted Development (no application needed)
- DNO connection in non-constrained area (G99 acceptance in 6 weeks)
- Standard equipment availability (no Tier-1 product shortages)
- Customer ready capex available (no financing approval cycle)
- Strong project management on the customer side
Five months is achievable but exceptional. Most projects benefit from a 9-12 month planning horizon.
What happens when timelines slip
The biggest schedule risk is DNO grid connection in constrained networks. Where the local DNO indicates G99 acceptance will take 12-18 months, options include:
- Stage 1 install at reduced output limit: Install full system but commission at reduced export, increase output when DNO grid availability allows.
- Behind-the-meter only: Install with zero-export protection devices, avoiding G99 process entirely.
- Wait for DNO upgrade: Some areas have committed DNO investment programmes that will resolve constraints within 18-24 months.
We work with the DNO from early proposal stage to understand the constraint position and recommend the right strategy.
Request a feasibility study including realistic programme assessment.
Critical path analysis: the dependencies that drive schedule
Project management of commercial solar requires understanding not just what needs to happen but what depends on what else. The true critical path — the sequence of dependent tasks that determines the earliest possible completion date — typically runs through DNO grid connection.
Critical path dependency chain for a 300 kWp office solar project:
- Feasibility and proposal (weeks 1-4) — cannot start until site visit scheduled
- Contract award (week 5-8) — depends on customer decision
- G99 application submission (week 6) — can only be submitted after DNO connection design is complete, which requires system design, which requires contract award
- G99 DNO response (weeks 6-22) — 45 working day statutory period, often 12-16 weeks in practice
- Equipment procurement (weeks 10-18) — can run in parallel with G99; 8-12 week typical lead time on Tier-1 equipment
- Planning approval (weeks 6-18) — can run in parallel with G99; Prior Approval is 56 days; full planning is 8-13 weeks
- Installation (weeks 18-26) — cannot start until G99 approved, planning approved, and equipment delivered
- DNO commissioning witnessing (week 26-27) — DNO visit required before grid connection can be activated
- Generation start (week 27)
The dependencies reveal why G99 is almost always the critical path: it cannot start until design is complete (requiring contract award), runs for 12-16 weeks, and installation cannot commence until G99 is approved. Everything else — equipment procurement, planning, detailed design — can run in parallel with G99.
How to compress the timeline
For project sponsors wanting to achieve faster commissioning, five actions materially reduce programme length:
1. Pre-application DNO enquiry. Before contract award, submit an informal enquiry to the DNO identifying the proposed connection point and indicative system size. Most DNOs respond to pre-application enquiries within 2-4 weeks and can flag any constraints, required studies, or connection cost contributions that would affect project economics. This eliminates surprises at formal G99 application stage and allows G99 submission immediately after contract award rather than 4-6 weeks later.
2. Pre-planning enquiry. Many local planning authorities offer Pre-Application Planning Advice (PAPA) services for commercial applications. For projects where planning route is uncertain (mixed heritage settings, Conservation Area adjacency, sites near residential properties), a PAPA response (typically 4-8 weeks, 500-1,500 fee) confirms the correct application route and identifies any key design requirements before full design investment. It eliminates planning refusal or amendment cycles later.
3. Long-lead equipment pre-order. Tier-1 solar panels (LONGi, JinkoSolar, Trina) have 8-12 week lead times from order to UK delivery. Where the project scope is well-defined at contract award, placing panel and inverter orders at contract award (week 5-8) rather than waiting for G99 approval can compress the overall programme by 4-8 weeks. Risk: if G99 approval is refused or major scope changes are required, pre-ordered equipment may need to be resold or respecified. This risk is typically small on well-designed projects in non-constrained network areas.
4. Customer-side decision speed. The table below shows how customer decision timing affects overall programme:
| Customer decision speed | Additional weeks added |
|---|---|
| Contract signed within 2 weeks of proposal | 0 |
| Contract signed in 3-5 weeks | +1-3 |
| Contract signed in 6-9 weeks | +4-7 |
| Contract signed in 10-12 weeks | +8-10 |
| Contract signed in 13+ weeks | +11+ |
This is rarely discussed in project management conversations but is frequently the largest controllable schedule variable. Customers with pre-approved capex budgets and streamlined internal approval processes consistently commission 2-3 months earlier than customers requiring full board cycle approval.
5. Commissioning scheduling. DNO commissioning witnessing visits require scheduling 2-4 weeks in advance in most network areas. Pre-booking the witnessing visit during the installation phase (rather than after installation is complete) eliminates a 2-4 week gap at the end of the programme. Most DNOs are willing to accept a conditional pre-booking with a 48-hour notice cancellation option.
COVID and supply chain impacts on 2026 timelines
The 2021-2023 supply chain disruptions significantly extended commercial solar timelines. Understanding what has normalised and what has not helps set realistic 2026 expectations.
Normalised by 2026:
- Shipping timelines from China (returned to 5-7 week ocean freight vs 12-16 weeks at peak congestion)
- Inverter supply (Huawei, Sungrow, Solis all have UK stock positions)
- Panel supply for Tier-1 manufacturers (significant capacity expansion means UK distributors hold substantial stock)
- UK installation labour (installation firms have rebuilt capacity post-COVID disruption)
Still extended versus pre-2020 baseline:
- Battery storage for large commercial systems (200 kWh+): Tesla Powerwall, BYD Battery-Box, Huawei LUNA all have 12-20 week lead times for large commercial orders
- Specialist switchgear (low-voltage switchboards, export limiting relay equipment): 10-16 week lead times for some configurations
- DNO response times in constrained areas (UKPN London areas remain 14-20+ weeks; this is a network capacity issue, not a supply chain issue)
For projects including battery storage, add 8-12 weeks to the procurement timeline versus storage-free projects. This is the most common cause of programme extension on 2026 UK commercial solar projects.
When the programme slips: managing stakeholders
Three communication steps that prevent programme delays from becoming stakeholder crises:
1. Issue a programme at contract award. The programme should show the G99 application date explicitly, the expected G99 response date, and the consequent earliest installation start. If stakeholders understand from the outset that G99 drives schedule, they are better prepared when the DNO takes longer than expected.
2. G99 progress update at week 8. If G99 has not been responded to by week 8 (midpoint of the statutory 45 working days), contact the DNO assigned case officer for a status update. Most DNOs are responsive to polite progress enquiries and will flag if their review is straightforward (pointing to fast resolution) or complex (flagging a possible network study requirement).
3. Communicate installation start date 4 weeks in advance. Building managers need 4 weeks to arrange tenant communications, parking management, lift access coordination, and security briefings for installation crews. Late installation start notification is one of the most common sources of client frustration on well-managed projects.
The fastest and slowest programmes: real-world benchmarks
Fastest completed programme: 4.5 months from enquiry to commissioning. Conditions: 80 kWp system (G98, no G99 required), Permitted Development (no planning application), equipment from UK distributor stock, single-occupier owner-occupier with pre-approved capex, experienced installation team.
Slowest completed programme: 24 months from enquiry to commissioning. Conditions: 850 kWp system on Grade II* listed building in Conservation Area, full planning permission plus Listed Building Consent required (both refused once, amended, resubmitted), G99 in constrained UKPN network area requiring network reinforcement contributions, customer financing approval requiring two board cycles.
Most commercial office solar projects of 100-500 kWp in non-heritage settings with non-constrained DNO connections complete in 9-14 months — closer to the fast end than the slow end of the distribution.
Key takeaways
- G99 DNO application is almost always the critical path for commercial solar; it cannot start until system design is complete and runs 12-16 weeks in most UK areas
- Pre-application DNO enquiry (before contract award) is the single most powerful schedule compression tool
- Customer decision speed is a significant controllable variable: sub-2-week contract turnaround versus 10+ weeks can shift commissioning by 2-3 months
- Battery storage adds 8-12 weeks to procurement timeline in 2026 — account for this in programmes including storage
- UKPN (London) remains the most constrained DNO network, with G99 response times of 14-20+ weeks common; NGED and SPEN are typically the fastest at 6-10 weeks