The National Trust is to carry out a refit of its Swindon headquarters after staff numbers more than doubled since 2005.
The headquarters will be refitted internally to bring all of the charity’s staff in Swindon under one roof, with 150 currently based in rented accommodation elsewhere in the town.
Staff numbers at the Swindon site have ballooned in recent years from 430 in 2005 to 850 today.
The charity, which declined to say how much the refit would cost, has also doubled the overall number of staff it employs over the same period to more than 6,000.
It said changes would include additional toilets, improved wifi, IT, lighting and drainage.
Staff would be moved temporarily before the refit begins, a spokesman for the charity said, but details were still being finalised.
The Swindon base won an award for sustainability from the Royal Institute of British Architects in 2006.
Tina Lewis, director of people and legal services, said: "To accommodate more people, some of whom are based at another office in Swindon, and to allow colleagues from across the country to be better connected, we are reconfiguring our Heelis office and upgrading our technology so it fits our needs now and for the future.
"At a time when employment in the area is facing considerable challenge, this investment also underlines the trust’s commitment to keeping its central office in Swindon."
The refit is expected to be completed by summer next year.