The two-tunnel ‘Hybrid+’ option for the long-awaited completion of the north-south corridor, the centrepiece of yesterday’s state budget, was spruiked as a solution that would save significant heritage items – including Thebarton Theatre and Queen of Angels Church – and hundreds of homes along the 10km route from the River Torrens to Darlington.

Premier Steven Marshall and Transport Minister Corey Wingard were today out spruiking the next stage of the “generational” plan – ground investigation works, ahead of a “reference design” that would inform a business plan to be reviewed by Infrastructure Australia and Infrastructure SA later next year.

But while the Government says up to 400 houses will need to be compulsorily acquired – far fewer than the 900-odd required under the rejected ‘open motorway’ option – affected residents won’t know until at least mid-next year whether their property will be among them.

“Community consultation is a huge part of this project,” Wingard said.

“We’ll be out there engaging with the community – we’re going to take them on that journey.”

But he conceded it would be some time before specific plans could be shared.

“We now move to the reference design,” he said.

“We started with the concept design, which gives us the bigger picture that the Treasurer outlined yesterday, then we do the drilling works, which were doing between now and the middle of next year to complete that reference design so we have that finer detail, so we can move on to that final design…

“That reference design will be part of formulating the final business case and that will go back to Infrastructure SA and Infrastructure Australia.”

While he said the decision to utilise tunnels would “save some 480 properties, which we think is really good for the community”, he noted the reference design “will drill down more specifically on the properties we’ll need to acquire and that won’t be till the middle of next year”.

He also refused to rule out any of the schools along the route – which include government-run Richmond and Black Forest primary schools and Warriappendi indigenous school – falling victim to the upgrade.

“Again, that’s back to the reference design, that’s where all that will be finalised,” he said.

“We’ve had to do these initial works, and what we’re saying here is we’re doing the planning to actually determine what’s needed and how that reference design will take shape, and that will be by the middle of next year…

“There are a number of schools along the journey – what we’re saying is we’re engaging with those schools and talking to them [but] until we have that reference design, that’s when we’ll know what it’s going to looks like [and] what properties we’ll need to acquire.”

Asked how the Government could then rule out demolishing a raft of contentious potential acquisitions including Thebarton Theatre, he said: “That’s because we’re doing tunnels.”

“That means where there’s a tunnel, it’s going under the Thebarton Theatre [so] we know for sure and certain that’s staying – we’ve made that commitment.”

There is one “heritage item” still under a cloud, a historic bunker adjacent Thebarton Oval.

“We’re engaging with that group… we are having a look at what that will mean and how significantly that will be affected,” Wingard said.

Local business owner James Franzon from the South Road Inner West Action Group said stakeholders had been told at a meeting with the minister last night it would be at least six months before they had clarity.

“There’ll be a reference plan but even then that wasn’t going to be 100 per cent definitive,” he said.

“It’s great that they’ve announced and committed to tunnels [but] there’s still unrest until we have that certainty of what’s going to be acquired and who’s going to be affected by that.”

He said already “you can see businesses starting to shift and more properties coming up for lease and sale” along the strip.

One of those is auto restoration business Rare Spares, which recently shifted from South Rd to Sir Donald Bradman Drive.

Owner Kahn Smith said the ongoing uncertainty around the roadworks was “definitely” a key factor in the move, along with a lack of carparking.

“We were right on South Rd – you step out the front door and you’re a metre from South Rd [so] the uncertainty of what roadworks were going to happen was an issue,” he said.

“There’s still a lot of unknowns.”

Opposition Leader Peter Malinauskas said while Labor was “providing absolute support for this project” and are “not going to be sniping from the sidelines”, it is nonetheless “a concern that major construction isn’t due to begin till 2023”.

“You’d hope that decision would be associated with a bit of detail so we can start to quell any community anxiety,” he said.