Formerly a derelict chapel, married couple Martin and Sue Stephenson bought the property in Snowdonia National Park for £27,000, and were living in Shrewsbury when they converted it into holiday lets.
They moved into the old vestry at the property in Abergynolwyn two years ago to enjoy the beautiful scenery.
But they now want to help the Alzheimer’s Society at a time when the charity is struggling to raise funds during the coronavirus pandemic.
Martin's father Ron, 88, has also been diagnosed with dementia and the couple hope to move to Yorkshire to be nearer to him.
The lucky winner of the prize draw would become the new owner of two furnished luxury homes, which include seven en-suite bedrooms between them, as well as a sauna, hot tub and baby grand piano.
"We bought it on a whim," said Martin, who is a retired building surveyor.
"We were in Wales and saw an advert for a chapel and decided to buy it. "We were living in Shrewsbury and going backwards and forwards to do the refurb.
"The chapel was starting to deteriorate so we gutted it and started again.
"The whole conversion took about four years. It was a real labour of love.
"We decided to move into the vestry two years ago to enjoy the amazing area of southern Snowdonia."
But he said the couple are now hoping to move nearer to family in Yorkshire.
Martin, 57, said: "My dad is suffering from dementia and we wanted to do something to help Alzheimer’s Society at a time when they are struggling to raise funds for their vital work.
"My mum and dad are 88 and live up in Sheffield and my mum is dad’s only carer.
"Dad was diagnosed with dementia just after we moved to Wales and we decided that we needed to be closer to them.
"We are a four hour drive to Sheffield from here, it's just too far.
"Instead of just selling, we hit on the idea of a prize draw to benefit the Alzheimer’s Society."
When the chapel was converted, two homes were created; Capel Jerusalem at the front of the building and Jerusalem Vestry at the rear.
The winner will be awarded both houses but for the property transfer to take place, a minimum of 330,000 ticket sales is required.
Martin says it takes into account the cost of running the promotion and a donation to Alzheimer's Society of £100,000.
If a minimum number of ticket sales is not achieved, the winner will be awarded a cash prize, with a percentage of ticket sales going to the charity.
The prize also includes land transfer tax and legal fees.
People will have until December 11 to buy their tickets and the raffle will be drawn at random on December 18.
Someone in the UK develops dementia every three minutes.
Alzheimer’s Society has worked closely with Governments worldwide, funders, industry and people impacted by dementia to support life-changing research.
The coronavirus pandemic has left many charities struggling and Alzheimer's Society is facing a 45 per cent fall in charitable income this year.
For more information and to buy a ticket for the raffle visit www.winadreamproperty.co.uk