Every canopy proposal tells you how many spaces the canopy will cover. Almost none tells you how many spaces you will lose. Those are different numbers, and for a car park that is busy, charged for, or barely adequate already, the second one decides whether the project is viable at all.
There are three distinct losses, and they arrive at different times from different causes. Only the first normally appears on a drawing.
Loss one: permanent, to columns and foundations
The columns have to stand somewhere. Where they stand depends on the structural format, and the formats differ enormously in how much parking they consume:
Cantilever. Columns sit behind the bays, with the deck projecting forward over them. Nothing stands between vehicles. In bay terms this is the most efficient format — often zero bays lost — which is exactly why it gets chosen where a layout cannot be altered. The cost is structural: the cantilever moment makes both steel and foundations heavier, so you pay for those bays in the frame instead.
T-frame. A single central column line carries a deck over two facing rows. If your car park already has a gap, kerb or planting strip between paired rows, the columns may fit into space that was never parking. If the rows abut directly, the column line has to come from somewhere.
Mono-pitch. One column line down one side. Against a boundary, verge or wall, it can cost nothing. Mid-car-park, it costs a strip.
The practical point: bay loss is a function of where your dead space already is. A car park with generous verges, planting beds and kerb lines can often absorb a column grid at no parking cost. A tightly optimised rectangle of bays cannot, and there the honest answer is either a cantilever or accepting the loss.
Ask for the proposed layout overlaid on your existing bay markings, with a count of bays before and after. If a supplier cannot produce that, they have not designed the layout yet.
Loss two: functional, to headroom
This is the loss nobody mentions, and it is often larger in practice than the column loss.
A canopy has a clear height under the beams. Any vehicle taller than that clearance cannot use the bays underneath — so those bays are still marked, still counted, and unusable for part of your fleet or your visitors. Sites where this bites:
- Vans and high-roof vans, which are a large share of trade and delivery parking.
- Refuse vehicles and service access, if their route crosses the canopy footprint.
- Ambulances and emergency access at healthcare and public sites.
- Minibuses at schools, care settings and sports facilities.
- Roof boxes and cycle carriers at leisure and retail sites, which generate complaints rather than a design problem, but generate them loudly.
Raising the clearance solves it — and immediately runs into two constraints. Height increases wind loading, which feeds back into every member and every foundation, so a high-clearance canopy costs more per bay across the whole structure. And in England the Class OA permitted development right caps overall height at 4 metres. Push past that and you lose prior approval and need a full planning application, which changes the programme as well as the cost. The conditions are set out on our solar carport planning permission page.
So the design question is not “how high can we build” but “which bays need to take tall vehicles, and can we keep those outside the canopy footprint”. Usually you can, if the question is asked before the layout is fixed rather than after.
Loss three: temporary, during construction
The biggest number, and the one most often left out of the business case entirely.
While the canopy is built, bays come out of use. You have two options and they price in opposite directions:
Phased. Works move through the car park in sections, each fenced, worked and released. Most bays stay usable. This is what an operating site normally needs, and it costs more and takes longer because plant is remobilised repeatedly into a constrained working area.
Closed. The car park comes out of use and the works run continuously. Cheaper per kWp and faster, and viable if you have overflow parking or a seasonal lull.
The only transparent UK figure for what that costs comes from DESNZ, which published an illustrative appraisal for an 80-space car park when it consulted on requiring canopies on new car parks. Assuming a three-and-a-half-month closure, it put foregone parking revenue at about £180,000 — against roughly £140,000 to install, about £28,000 a year of energy savings, about £22,000 a year of EV charging revenue and about £1,400 a year of maintenance.
Read that again: in the government’s own worked example, the parking lost during construction was the largest single figure in the appraisal, larger than the canopy. The proposal to mandate canopies was not taken forward — confirmed on 21 May 2026 — but the appraisal remains the clearest public illustration of the trade-off, and the lesson survives the policy. A cost model that stops at the capital figure has omitted the biggest line.
Putting a number on it for your site
A rough sequence that gets you an answer without a survey:
- Count your genuine dead space — verges, planting, kerb strips, unmarked margins. That is your column budget.
- Identify which bays must take tall vehicles and mark them as outside the canopy footprint.
- Decide phased or closed, and cost the parking you lose either way at your own charge rate or your own operational cost of not having it.
- Then size the canopy over what is left, using about 1.7 kWp per bay as a planning figure — the derivation is on our car park solar canopies page.
Doing it in that order sometimes shrinks the canopy. That is a better outcome than discovering the constraint after the order is placed, and it usually improves the business case rather than damaging it, because the bays you drop are the awkward ones that would have driven the structural cost up.
The wider cost hierarchy — foundations first, then format and height — is on our commercial solar carport cost page, and the build sequence, including how phasing affects the programme, is on our solar panel canopies installation page.
Want the bay count done properly before you commit? Request a free feasibility assessment — send the bay layout and dimensions and it comes back by email.