B.C. Premier Christy Clark is giving NDP Leader John Horgan and Green Party Leader Andrew Weaver until Saturday to provide advice on how the government should proceed on the $9 billion Site C dam.

In similar letters addressed to the two opposition leaders, Clark explains the potential implications of delaying relocating two homes slated to be knocked down for dam construction.

"In your response, I would ask you to specifically indicate," read the letter, "whether or not you maintain your request to delay the relocation of the two houses in question with the knowledge that BC Hydro ratepayers may be at risk of incurring a $600 million cost increase to Site C."

Ken and Arlene Boon stand outside the house her grandfather built in the 1940s. The pair have to leave the home to make way for the Site C dam. (Andrew Kurjata/CBC)

The letter indicates the relocation of the two homes is part of a two-year road-and-bridge project needed for river diversion to occur in September 2019.

Clark says BC Hydro indicates that "should river diversion not be completed as scheduled, a year-long delay would occur in order to divert the river during the next low-flow period, September 2020."

She maintains such a delay would lead to the $600 million in cost overruns.

Clark is also giving the opposition four days to decide whether it would like the government to issue a "tools down" request to BC Hydro — the premier arguing in the letter, the NDP's proposed B.C. Utilities Commission review would create detrimental delays and any request to Hydro would need to occur soon:

"...given that the project is likely to progress past the "point of no return" before the BCUC review you have suggested could be reasonably concluded."

Bob Peever, BC Hydro's Site C construction manager, explains the scope of the project to reporters on April 19, 2017. (Richard Zussman/CBC)

Premier's letter in response to Horgan

Clark's letter is in response to a letter sent to BC Hydro CEO Jessica McDonald last week from Horgan. In that letter, Horgan urges Hydro not to finalize any contracts that do not contain a penalty-free cancellation clause, until a new government has the confidence of the legislature.

Horgan is also advising Hydro to suspend the evictions of two homes impacted by Site C construction "until the future of Site C is firmly determined."

Green Party Leader Andrew Weaver has responded to the latest letter with a letter of his own, saying he is unsure of the claim that delaying evictions could cost $600 million.

"Before I can comment on these assertions, I require access to the supporting evidence, including but not limited to the signed contracts, the project schedule and the potential alternative project timelines that could allow an independent review to be conducted at minimal cost to the ratepayer," said Weaver.

Questions around point of no return

There are also questions about whether the BCUC review would push the project past the "point of no return."

The chair of the original Site C review panel, Harry Swain, says it likely wouldn't.

Swain says Hydro has spent about $2 billion so far and would have to spend much more, perhaps as much as $6 billion, for it to be past that point.