Earlier this year, the cost of preparing the planning application for the Lower Thames Crossing had already topped £300 million. And every penny of that came out of the public purse.

Add to that the 350,000-plus documents produced by various government departments over the course of its planning and consultation saga - one of the biggest in British planning history - and it means the crossing has become something of a behemoth before a spade has been sunk into the ground.

A decision on whether the government was to give it the go-ahead was expected from Transport Minister Louise Haigh on Friday - but has now been pushed back until next May.

We had expected a decision earlier this year but the general election in the summer saw it kicked into the autumn.

So just what exactly is it, why is it deemed so important to the county and why, at an estimated £9 billion, is it so incredibly expensive? Here’s our guide…

What is the Lower Thames Crossing?

A 14.3-mile stretch of road, it will provide a link off the M2 near Gravesend, under the River Thames, and emerging near Tilbury where it will eventually link up to the M25.

The project will see a junction created on the M2/A2 and then take motorists to the east of Gravesend on a road to be called the A122. The tunnels would be created just east of the village of Chalk.

There would then be two tunnels - one northbound, one southbound - which will run beneath the Thames.

At 2.6 miles in length, they would be the longest road tunnels in the UK. By way of comparison, the Dartford Tunnels are less than one mile long, as they cross the Thames at a narrower part of the river.

Once you’ve emerged into Essex, you join the A13 in Thurrock and a new road which will then connect to Junction 29 of the M25 in the London Borough of Havering.

The project is being driven forward, pardon the pun, by National Highways - the government department which oversees our major road network.

Will it be another toll road like the Dartford Crossing?

Oh yes. Make no mistake, if it gets built and you end up using it, you will be paying for the privilege.

Quite how much remains to be seen. The Dartford Crossing is a relatively affordable £2.50 (or £2 if you have an account). But then the toll, which was designed to be dropped when the construction costs were repaid, has continued ever since.

But costs for road use have changed a great deal since then. So, for example, the M6 toll road - which allows users to escape the worst of the congestion around Birmingham - is owned and operated by a private firm. Each stretch of the 27-mile route costs one-off users an eye-watering £9.70.

Don’t expect the Lower Thames Crossing to hit those sort of heights… but don’t expect to be as cheap as the Dartford Crossing either.

Oh, and it will likely have ‘free-flow’ tolls too - which means you’ll need to pay in advance or within a certain time frame.

Why is there a need for the crossing?

If you have experienced the Dartford Crossing, the answer is probably somewhat obvious. The initial tunnel, which opened in 1963 and was then joined by a second tunnel in 1980, took all traffic under the Thames until 1991 when the QEII Bridge opened.

It enhanced capacity with both tunnels now operating northbound; southbound traffic uses the bridge. Your closest option if you want to get a little closer to London is the Blackwall Tunnel which, from next year, is likely to be tolled at peak times.

Carrying more than 50 million vehicles a year, the Dartford Crossing is designed to handle 135,000 vehicles a day, but the average is now 150,000 a day – with the busiest days seeing up to 180,000 vehicles using it.

In short, it creates a classic pinch point for both cars and lorries looking to reach anywhere north of the Thames. And given Kent’s gateway to Europe status, delays for freight can prove highly costly.

So for many years, there have been talks of creating another crossing to alleviate the pressure… hence the Lower Thames Crossing.

What will Kent get out of it?

There’s always an argument to say that Kent’s biggest transport infrastructure projects are designed to get people through and out of the county as quickly as possible. Think HS1 or our motorways.

But that would be to overlook the need of the many hundreds of thousands - if not millions - of people living in the county who need to access other parts of the country for work or leisure. And, most importantly, the freight which arrives at our ports and needs transporting, as quickly as possible, by our road network to elsewhere on this island of ours.

The county’s inward investment agency, Locate in Kent, says the crossing would be a “game-changer” to the local economy. After all, heavy and light goods vehicles currently account for around a quarter of all Dartford Crossing traffic, according to National Highways.

Increasing the capacity of driving goods out of the county means the green light for the potential expansion of our ports. It would be welcomed too by those behind Manston Airport’s plans as a cargo hub.

Once complete, it would provide better access to Essex and London - enhancing Kent’s appeal to businesses and house-buyers.

For those living in the east of Kent, the ability to reach north of the Thames will be far quicker and - at first at least - far less congested.

However, critics suggest the economic benefit may, realistically, not compare to the cost of building the route. Not to mention the fact some cite just five years before both crossings find themselves equally as busy as Dartford does today.

How was the route for the Lower Thames Crossing chosen?

Back in 2009, the Department for Transport commissioned a study into the best options to “address capacity constraints” at the Dartford Crossing.

It drew up five options: Option A - Additional capacity at the existing Dartford Crossing; Option B - Swanscombe Peninsula link A2 to the A1089; Option C - East of Gravesend and link to the M20; Option D - M2 link to Canvey Island; and finally Option E - Isle of Grain link to east of Southend.

Three years later, this list was whittled down to just three and refined.

So by 2013, three options were put out to public consultation: Option A: at the site of the existing A282 Dartford-Thurrock crossing; Option B: connecting the A2 on the Swanscombe Peninsula with the A1089; Option C: connecting the M2 with the A13 and the M25 between junctions 29 and 30. There was also an additional Option C which would include the widening of the A229 between the M2 and M20.

Additional work at Dartford was ruled out due to the existing issues and pollution the link creates, while the Swanscombe Peninsula link was dropped due to the planned London Resort theme park which was being heavily pushed at the time. Why build there and miss out on the chance of an all-singing, all-dancing entertainment complex? If only they knew…

All of which left Option C - east of Gravesend.

There were plenty of opponents, as you can imagine, to the road slicing through countryside, not to mention fears it would, ultimately, become just as rammed as the Dartford Crossing.

It was officially confirmed as the preferred route in April 2017. Then Transport Secretary Chris Grayling declared it would create 6,000 jobs and boost the British economy by £8bn. It was immediately welcomed by business groups in the county and the likes of the Port of Dover.

How much will the Lower Thames Crossing cost to build?

Major road infrastructure projects never come cheap. So by the time you’ve added in the engineering challenges of the longest road tunnels in Britain to the equation and inflation, the price was only ever going to go up.

When the route was originally announced back in 2017 the cost was expected to be around £5.3bn. Today that number is £9bn.

It would come as very little surprise if that figure becomes bloated by the time work begins in 2026 and vehicles finally take to the road if it opens, as planned, in 2032.

So far, putting in the Development Consent Order alone - the key planning permission for work to begin - has cost £300m. Buying land has added more than £110m, specialist land surveying and investigation, some £300m. Those figures will, one assumes, form part of the £9bn total cost. Or, at least, you’d like to think so.

How can the government afford £9bn when it keeps saying it’s got no money?

Good question. Chancellor Rachel Reeves has mentioned that “£22bn black hole” left by the Tories so often it has become something of her catchphrase.

But don’t expect every major infrastructure project to be stopped in its tracks. If this government is to boost the economy, as it keeps vowing, it will need to invest. And there’s an argument to say boosting a key trade route would be a good way of achieving that.

However, there’s plenty of debate that says the billions the crossing would generate may not be borne out in reality.

All of which means there has been much talk of the government pursuing a Private Finance Initiative for the Lower Thames Crossing.

This effectively sees the private sector shell out the bulk of the money for its construction with a view to clawing back that cash - and make a profit - over the long term.

So, for example, the government enters into a contract with private investors to raise the money and then promises them returns - most likely through toll charges - for a fixed period of time. Expect that to be decades rather than years.

The road would remain public - just financed differently to most others.

There is also a compelling argument that suggests so much public money has already been invested in the Lower Thames Crossing - just to get it to this point - do you want to literally throw that all away?

Is the planning document really 359,000-plus pages long?

Well, there is some dispute about this. The actual document which supported the Development Consent Order application is, according to transport campaign group Transport Action Network (which is opposed to the project but painstakingly counted every page) is only a rather modest 63,330.

It says the 359,000-page claim was touted by Britain Remade - a group calling on major investment to boost Britain’s economy - and has gone unchallenged since.

Transport Action Network says the inflated figure is derived by adding all of the evidence submitted by National Highways during the six month-long examination, which includes “many duplications and rebuttals of evidence”. Something National Highways has subsequently backed up.

Either way, suffice to say, it’s a long document which, realistically, probably only a handful of people have read from start to finish.