By Clare Jim

HONG KONG, Dec 3 (Reuters) - Sunac China 1918.HK is seeking approval from onshore bondholders on a sweetened debt restructuring plan, according to a private filing seen by Reuters, with a voting deadline on Monday.

In a bond document to creditors, Sunac offered 1% upfront cash plus accrued interest by the end of 2025 for the debt extension option, and raised the buy back price for existing bonds to 20% of the nominal values from 18% proposed in the initial offer.

Sunac is aiming to deleverage its 15.4 billion yuan ($2.12 billion) outstanding onshore debt by more than half through the restructuring exercise, making it the first Chinese property developer to carry out a steep debt cut.

Most developers have only extended their onshore bond maturities so far - some by twice or three times - instead of implementing a debt restructuring.

"The time length and level of the company cashflow being under pressure far exceeded expectations when we passed the bond extensions in late 2022," the developer said in the document.

"That's why we have to urge bondholders to support this bond restructuring proposal."

Sunac added it has yet to secure new financing since its last restructuring while its property sales remain under pressure.

The Beijing-based developer first restructured its onshore bonds in late 2022 by extending all coupon and principal payments. After several payments became due again, it started delaying payments in June.

Sunac declined to comment.

The company said in a public filing last month it was offering four options to mainland China creditors, which include one that swaps debt for economic income rights to its Hong Kong-listed shares, also a first for an onshore debt restructuring.

Other options are accepting repayment from the planned sale of one land parcel, and replacing existing notes with notes with a 1% coupon and maturities as long as 9.5 years and which also stipulate that the first principal payment would start only in the fifth year.

Once one of China's top developers, Sunac was the first in the sector to complete a successful restructuring of its offshore debt since the present debt crisis began in mid-2021.

($1 = 7.2681 Chinese yuan renminbi)

(Reporting by Clare Jim; editing by Jason Neely)

((clare.jim@thomsonreuters.com;))