The stately home has for months rumoured as the venue for the 32-year-old's nuptials to art dealer Casper Jopling on Saturday.
Images show people building various structures within the grounds, including a huge marquee next to the Atlas Fountain.
-> Demand for water in Yorkshire 'set to outstrip supply by 2035 unless action is taken' as climate change takes a tollOthers could be seen pitching two areas of teepee tents - one within the Rose Garden and one just next to it.
There were workers sorting through deliveries of urinals, trimming back foliage throughout the grounds and fitting lights within the main marquee complex.
Piles of large wicker baskets and other wicker furniture can been seen, fuelling rumours the wedding will have a festival theme.
It has been suggested the couple's impressive reception could play host to some A-list guests - Ellie is a close friend of Prince Harry and she sang at the wedding reception of Prince William and Kate Middleton.
-> Yorkshire's devastating record of cruelty to birds of preyCaspar is reportedly inviting close friends Princesses Beatrice and Eugenie and others on the guest list could include Ed Sheeran and Rita Ora, according to the SWNS news agency.
The couple are thought to have chosen the Yorkshire venue because of Casper's strong links with the county.
The 27-year-old - who Ellie has been dating since 2017 - is from an aristocratic Yorkshire family, who live at Frickley Hall, near Doncaster and is the son of landowners from the region.
His father is the Honourable Nicholas Jopling, who comes from a family of farmers in the Thirsk area and his grandfather is the former MP Michael Jopling, who served as Deputy Lieutenant of North Yorkshire.
Although Castle Howard representatives have previously refused to be drawn into the singer's plans, the visitor attraction's website confirms the house and grounds will be closed to the public on Saturday for a 'private event'.
There are rumours the ceremony will take place at York Minster which is due to close at 12.30pm on Saturday rather than 4.30pm as is usual, said the agency.