Office building solar — the 12-month timeline from feasibility to commissioning

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.