Cantilever vs Multi-Bay Solar Carport: Structure Guide

Two structural types dominate UK solar car-park canopies: the single-post cantilever and the multi-bay (back-to-back, continuous) frame. They generate the same electricity per panel and use the same 450W modules, but they sit on different foundations, park differently, and cost meaningfully different amounts per kWp. Get the choice wrong and you either overspend on steel you didn’t need or lose parking bays you couldn’t afford to lose. This guide compares the two honestly, with real UK 2026 numbers, so you can brief an installer on the right structure before you ask for a quote.

The two structures in plain terms

A cantilever carport is supported by a single row of posts down one side of the parking bay. The canopy arm reaches out over the cars from that one column line, so the bays themselves are completely column-free. Nothing stands between the parked vehicles. It behaves like a diving board: the load is carried back to one anchored edge.

A multi-bay carport (also called back-to-back or continuous) uses columns positioned between rows of parking — typically down the centre of a double row, or along both edges of a single run. Because the roof is supported from more points and shorter spans, it needs less steel per square metre and less foundation depth per kWp. The trade-off is that those support columns sit within or beside the parking area.

Both are built as turnkey systems — steel structure, PV, electrical, and DNO connection under one contract — not bare frames. Both use galvanised steel in almost all commercial cases (timber/glulam is a premium, heritage-only alternative that raises the per-kWp cost regardless of layout).

Head-to-head comparison

Factor	Cantilever (single-post)	Multi-bay (back-to-back / continuous)
Support columns	One row, along one edge only	Columns between/within rows
Bays under canopy	Fully column-free	Columns sit near or between bays
Cost per kWp (commercial scale)	Upper end: £1,000–£1,400/kWp	Lower end: £900–£1,200/kWp
Cost per bay	Typically £8,000–£12,000	Typically £6,000–£10,000
Steel + foundations share of cost	Higher (long cantilever spans need heavier sections)	Lower (shorter spans, lighter sections)
Best span / scale	Small–medium runs, single rows	Large runs, double rows, 50+ bays
Parking usability	Easiest — no columns to manoeuvre around	Good, but columns constrain some bays
Foundation type	Deeper/larger to resist overturning	Lighter per column; ground screw common
Best-fit layout	Tight sites, edge-of-lot, premium/customer-facing	Open, regular, large employee/retail car parks
Typical yield	~2 kWp per standard bay (4–6 × 450W)	~2 kWp per standard bay; up to ~4 kWp double-sided

The cost gap is not marketing — it’s physics. Steel and foundations make up roughly 45% of a canopy’s total cost. A cantilever carries its entire roof load back to one column line, so those sections and their foundations have to be bigger to resist the overturning moment. A multi-bay frame breaks the same roof into shorter, supported spans, so each member is lighter and each foundation smaller. Spread across many bays, that structural saving is where the cheaper cost per kWp comes from.

Cost and scale: why bay count matters

Because steel and foundations are a fixed-ish share of every project, cost per kWp falls as bay count rises — and it falls faster for multi-bay frames. At commercial scale, elevated solar canopies run £900–£1,400/kWp; smaller or more complex jobs sit at £1,200–£3,000/kWp. (For context, rooftop solar is cheaper at £700–£1,050/kWp because it needs no dedicated structure — you’re paying for the parking canopy as well as the generation.)

Sizing is straightforward: a standard bay carries about 2 kWp (four to six 450W panels), so 100 bays is roughly 180–270 kWp. Double-sided designs push up to ~4 kWp per bay. At UK yields of 900–950 kWh/kWp (bifacial modules add 5–12%), a 200 kWp car park generates around 180,000–190,000 kWh a year — worth far more consumed on site (self-consumed solar is worth roughly twice exported). DESNZ modelled an 80-space car park saving about £28,000/year through self-consumption, and the Princess Royal Hospital in Telford has a 200 kW canopy expected to save around £35,000/year from early 2026.

Payback lands at 8–12 years solar-only, tightening to 7–11 years with EV charging added. It will never be a rooftop-style 4–6 years — you’re funding a structure as well as the panels — but the structure buys you covered, revenue-generating parking. Full figures are on the cost page.

Parking usability and span

For anyone who actually parks under the canopy, the cantilever wins on usability. Column-free bays mean no manoeuvring around posts, no door dings against steel, and cleaner sightlines — which is why cantilevers suit customer-facing and premium car parks where parking experience matters. See our workplace and office car-park canopies and retail/customer car-park work for typical layouts.

Multi-bay structures are perfectly usable — most large UK car parks already have lighting columns and level markers — but a few bays will sit tight against a support. On big, regular, open sites that trade-off is minor and the per-kWp saving is large, which is why multi-bay dominates high-count employee and retail car parks.

Span is the other divider. Cantilevers are efficient over single rows and short-to-medium runs; push the reach too far and steel weight (and cost) climbs sharply. Multi-bay frames handle long, wide, double-row runs comfortably because each span is short.

Everything else is identical

Both structures share the same UK rules and engineering:

• Planning (England): Class OA permitted development, in force since 21 December 2023, covers non-domestic off-street parking. It’s prior approval (siting, design, glare) — not full planning — for structures ≤4m high, more than 10m from any dwelling, excluding listed buildings and scheduled monuments, with a SuDS condition, started within three years. England only — Wales, Scotland and NI need standard planning. Height and layout affect both types equally.
• Structural design: Eurocode 1 (BS EN 1991) wind and snow loading, foundations by ground screw (~90% of sites), ballast or driven pile, under CDM 2015 and BS 7671.
• Grid: G98 up to 3.68kW/phase, G99 above (most commercial) — allow ~4–8 weeks. MCS certification is required to claim the Smart Export Guarantee.
• EV: either canopy powers 7–22kW AC charging plus lighting; neither runs standalone 50kW+ DC rapids without a grid upgrade and battery. See EV-charging solar canopies.

Funding is also structure-agnostic. Businesses use the £1m Annual Investment Allowance and the 50% First-Year Allowance — solar is special-rate plant, so it’s excluded from full expensing. The SEG pays roughly 1–15p/kWh for export, the England business-rates exemption runs to 31 March 2035, and the Workplace Charging Scheme (to 31 March 2027) covers up to £500/socket. NHS and schools can access GB Energy capital; Salix offers 0% loans. Full detail — including PPA (zero upfront, off balance sheet) vs capex ownership — is on the grants and funding page.

Choose X if… / choose Y if…

Choose a cantilever (single-post) carport if:

• Parking experience matters — customer, visitor or premium/showcase car parks
• Your site is tight, irregular, or edge-of-lot where you can only anchor one column line
• You have single rows rather than large double-row blocks
• You’ll accept the higher £1,000–£1,400/kWp (roughly £8,000–£12,000/bay) to keep bays fully column-free

Choose a multi-bay (back-to-back/continuous) carport if:

• You have a large, open, regular car park — think 50+ bays, double rows
• Lowest cost per kWp is the priority (£900–£1,200/kWp, roughly £6,000–£10,000/bay)
• A few bays sitting near a support column is an acceptable trade-off
• You want maximum generation per pound across a big employee, retail, school or NHS/public-sector site

A realistic middle path: many UK car parks mix both — cantilevers over the customer-facing frontage and multi-bay runs across the bulk employee area. A good installer will model both against your bay count, and you’ll usually find the per-kWp saving of multi-bay is decisive above ~40–50 bays, while cantilever earns its premium wherever the parking experience is on show.

The bottom line

There is no universally “better” structure — there’s the right one for your car park. Cantilever buys column-free, premium parking at a higher cost per kWp. Multi-bay buys the cheapest generation per pound at scale, at the cost of a few constrained bays. Both are engineered to the same UK standards, qualify for the same funding, and pay back in the same 8–12 year window (7–11 with EV charging).

Send us a site plan and bay count and we’ll size both options — turnkey, MCS-certified, structure plus PV plus electrical plus DNO under one contract. Start with a quote, or call +44 7707 970661.