The whole community is invited to share in the celebrations at Voluntary Action Rutland on Friday next week.
The £900,000 building, which stands behind the existing volunteer centre in Barleythorpe Road, will help the charity to meet increasing demand for its services.
It was made possible partly by a buy-a-brick scheme which was supported by well-wishers from across the county and beyond and raised £300,000.
Director Lindsay Henshaw-Dann said: "This community resource would not be here today without the amazing support and efforts of local people participating in buy-a-brick appeal, baking cakes and holding fundraising events.
"Thanks are also extended to Rutland County Council for its significant gift of land upon which the extension is built. And due to the generosity of Rutland Agricultural Society parking is available at the back of the building."
Many community groups and charities use the facilities at Voluntary Action Rutland as well as being supported by staff and volunteers at the centre.
The social car transport scheme and Age Concern are already based in the new building and among those who will also use it are Rutland Access, Vista, Rutland Supported Youth Group, the Parkinson's Disease Society and the arthritis group.
It has a board room that can take up to 20 people, a meeting room with capacity for 75 people with kitchen, internet and interactive white board connections and 100 car parking spaces.
It is fully accessible to wheelchair users and the walls have been decorated in a way designed to ensure visually impaired people can navigate the building easily.
Sketches of the new building were first drawn up in 1999, planning permission was approved in 2003 and work started on the building in January last year.
The first phase of the development cost £350,000 and with problems such as rising steel prices pushing up the total cost the buy a brick scheme was launched in April, 2007.
Bricks cost £1 each, a row was £5 and a virtual wall cost £100 and many fundraisers took part in house to house collections.
The building also received grants from East Midlands Development Agency and Welland Sub-Regional Strategic Partnership and Voluntary Action Rutland paid more than £100,000 from its own fundraising and resources.
RAF Cottesmore raised cash from a run around Rutland Water last year and Navitron of Oakham donated a solar panel that now heats the water.
The open day is open to everyone from 10am to 2pm on Friday. As well as tea and cake there will be tours of the building and the opportunity to view the facilities.
Organisations based at the centre and occasional users will also have stalls and promote their services.
Mrs Henshaw-Dann said: "We want people to come along and have a sticky bun and a cup of tea so that we can say thank you for all their efforts."