A few weeks back, we reported that London’s newly elected mayor Sadiq Khan had decided to go ahead with the controversial Garden Bridge project.

The reason he gave was that the project was so far advanced that the taxpayer had already spent £37.7m, much of which would be recouped if the Bridge was completed. Consequently, it'd cost twice as much to cancel the project as to finish it.

This raised many questions, but perhaps the biggest was – how on earth has anyone managed to spend £37.7m already?

Well, now we know. Earlier today a statement from the Garden Bridge Trust, the charity promoting the scheme, popped into my inbox, outlining the money spent so far. (We've included the lot, in the name of completeness, but highlighted the key points so you can skim read.)

Costs are as follows:

1. Pre-planning (up to the point at which the Garden Bridge Trust took control of the project from TfL) – design, preparation of planning application materials by specialist technical consultants, public consultations. £9.7m (27 per cent).

2. Pre-construction activities – progressing the design; obtaining licenses, permits and planning approvals (including stakeholder and community consultation) for detailed plans, for example the Construction Logistics Plan, Code of Construction Practice, operations and security plans. Other activities include selection and tagging of trees and plants, river survey and ground investigation works, procurement of the construction and landscaping contractors, procuring and placing orders for materials. £22.7m (63 per cent).

3. Professional services – legal, property & planning advice. £3.4m (10 per cent).

That doesn’t quite add up to £37.7m, so there’s also this bit:

Of the £36.4m received, the total cost of public funding spent so far is just under £36m. This excludes £1.3m of liability. This was a figure allocated for costs incurred if the project was stopped for any reason.

It was part of the figure released by the mayor at mayor’s Question Time two weeks ago, and is funding that has been allocated but remains unspent.

So to sum up, that's £9.7m on pre-planning before the GBT was set up, and £3.4m on assorted professional services, but the lion's share of the costs – £22.7m – is the detailed plan to make the thing happen.

Oh, and there's some money that hasn't been spent, but will be – "if the project is stopped for any reason".

Major infrastructure is expensive, for all sorts of reasons. And while it's easy to sneer at the idea you could spend £37m without actually building anything, getting to the point where you can build something costs a lot of money.

As a layman, though, it's difficult to know what kind of figure is reasonable. So I forwarded the press release to a consultant who works on major urban projects. They were sceptical that the programme described would cost nearly £38m:

I was involved in a £125m project and the absolute maximum that we were allowed to spend before virtually everything was signed off (land, planning, funding, internal approvals, etc.) was £8m. And that wasn't an organisation known for its financial restraint.

The consultant was particularly bemused about this bit:

Other activities include selection and tagging of trees and plants, river survey and ground investigation works, procurement of the construction and landscaping contractors, procuring and placing orders for materials.

Spending money on contractors and supply chain – before a project is definitely going ahead – locks in yet more spending. That’s why there’s money set aside to be used “if the project is stopped for any reason”. Our consultant concluded:

In truth, they have not discharged all of their planning conditions, they don't own the land yet, and they don't have all the funding. They never should have been allowed to sign those contracts.

I put all this to the Garden Bridge Trust who, unsurprisingly, disagreed. Here's a spokesperson:

We've got planning permission from Westminster and Lambeth. There are a couple of things to be discharged, but they're very minor.

You don't just wave a wand and produce a bridge. You have to do very detailed work.

In other words, the bridge is going ahead, so it’s right and proper to spend the money.

The Garden Bridge, and the public money paying for much of it, is an issue that seems to get people fired up. This probably isn't the last time this debate will rear its head.

Still, national treasure Joanna Lumley is happy, that's the important thing.

Jonn Elledge is the editor of CityMetric. He is on Twitter, far too much, as @jonnelledge.

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