The centre hopes the restoration plans will be completed within two years
A campaign is being launched to raise £200,000 to ensure plans to revive the national language and heritage centre in Gwynedd can be completed.
The Nant Gwrtheyrn site on the Lleyn Peninsula has already secured £3.5m in grants to redevelop the former quarry village.
But supporters say the extra cash is needed to make sure the final phases of the planned improvements go ahead.
Plans include upgrading accommodation, teaching facilities and access.
The village itself is tucked away in a hidden valley on the coast, and was only accessible by a winding single-track road along cliffs, which the centre's staff themselves described as an "hair-raising experience".
Improvements to the road were the first phase of plans for the centre, and were completed in 2008.
Work is also due to be completed this year on upgrading the terraced house accommodation in the village.
It's good to be able to launch an appeal which is an attempt not to salvage something... but rather to take full advantage of success
Nant Gwrtheyrn Trust
The former quarrying cottages are being transformed from what has been described as "cosy accommodation" to modern facilities that include family and double rooms, for 77 guests.
But full funding to improve the former quarry manager's home and a second row of terraced houses for teaching use is still being sought.
Discussions are also underway on plans to expand the site's heritage centre, and building a new reception and administration office for the site.
Around 25,000 people have attended courses at the Nant since opening its doors to Welsh learners in 1978.
Jeff Williams-Jones, chair of the centre's trust, said: "It's good to be able to launch an appeal which is an attempt not to salvage something, but rather to take full advantage of success and of support which has already been secured."
The appeal for funds is being backed with a pamphlet outlining the history of Nant Gwrtheyrn and the plans, which will be sent to supporters.
The centre is also pledging to record the names of substantial donors in the names of the refurbished houses and furniture.
"The Nant, in its new form, after the work has been completed, will provide work for 32 local people within one of the most Welsh-speaking areas of Wales - but an area where job opportunities are scarce," added Mr Williams-Jones.
"We are confident of a strong response."