The owners of Penrhyn Quarry want to expand the site in a bid to extend the lifetime of the Gwynedd quarry.
Quarrying has taken place at the Bethesda site since the 16th century with major development starting in the 1770s - later becoming the world's largest slate quarry.
The historic quarry still employs 170 workers with enough quality slate reserves to continue at the site for at least 10 years.
Now a scoping report has been submitted to expand the quarry in a move than would increase reserves from just over six metric tonnes of slate to almost nine metric tonnes.
This could add another five years to the site's operational life.
The plan has the support of the Unite union who said it would give greater job security for workers.
The scoping report from SLR Consulting Limited states: "The prospective applicant is minded to submit a planning application for a small lateral extension to the quarry workings. The proposed extension would be contiguous with the current working area, located at the northwestern corner.
"Based on the latest geological modelling of the approved quarry development scheme, reserves of good quality slate (for roofing and aggregates) is estimated at 6.3Mt; this takes into account the reserves lost as a result of the substantial dyke which cuts across the workings.
"This reserve is only sufficient to sustain production for around 10 years. By extending the quarry workings as set out in this document, the reserves of good quality slate increases to 8.9Mt."
Penrhyn Quarry itself extends over an area of some 318 hectares.
The report submitted to Gwynedd council adds: "The proposed development of the extension represents a continuation of the existing quarrying operations. The quarry has been in existence for a number of years and currently employs 170 personnel directly.
"This employment provides indirect effects through increased spending in the area on goods and services."
Welsh Slate was sold earlier this year to UK based Breedon Group.
Unite's regional officer Paddy McNaught said: "We welcome this move by the quarry, it is positive to see that the company is investing in the future of Penrhyn Quarry, this gives greater security to the workforce.
"Since taking over Breedons have invested in the site to secure its future. As a union we are supportive."
