Engineering Recommendation G99 governs the connection of generation equipment (including commercial solar PV systems) above 16A per phase to the UK distribution network. Projects below this threshold use the simpler G98 fast-track process; above it, G99 is mandatory.
The G99 process has two stages. Stage 1 is the application to the DNO with system specs, location, and intended capacity. The DNO assesses available network capacity and issues a quote, this can take 8-12 weeks for projects under 1 MW, longer for larger. Stage 2 is acceptance, payment of any DNO works costs, and scheduling of the connection.
Connection costs vary enormously by DNO area and local network capacity. A small commercial rooftop with spare capacity at the existing supply point might cost a few thousand pounds. A multi-MW ground-mount in a constrained area might require £100k-£500k+ of network reinforcement (transformer upgrades, new cables, new substations) before connection is possible.
The biggest variable is your DNO. UK Power Networks (London, East, South East) and National Grid Electricity Distribution (Midlands, South West, South Wales) have wide variation in available capacity. Some areas, particularly across Devon, Cornwall, Somerset, parts of Wales, and the South Coast, are heavily constrained, with multi-year wait times for any new generation export connection.
A growing trend is "active network management" (ANM) connections, accepting curtailment in constrained periods in exchange for a faster, cheaper connection. For solar, where peak generation often coincides with grid stress, this can work well economically.