The public will have a chance to sticky-beak inside the city's architectural gems during Open House Bendigo in October.

Buildings in Castlemaine and Harcourt will be added to the architectural trail in 2019 as the event grows.

Open House Bendigo program and business manager Victoria Bennett said the event was such a great experience in 2018 the organisation decided to run it again.

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Ms Bennett said major building works and developments in Bendigo meant Open House knew it could return and expand the program, allowing it to do a lot more.

She said visitors were not only interested in Bendigo's spectacular heritage buildings, but also in how they were used today. Ulumbarra Theatre, a heritage site transformed into a new use, was one example of this.

"We really felt like there was a real desire for people from Melbourne to travel up but also people from Bendigo to look around," Ms Bennett said.

"It was a huge success from our point of view. We really were overwhelmed by just how well it was taken.

"We felt that people were really engaged and excited to get out and actually get inside some sites they hadn't normally had access to."

City of Greater Bendigo Manager Tourism and Major Events Terry Karamaloudis said new visitors meant more money in Bendigo's economy.

These visitors spent money in Bendigo's restaurants, cafes and bars, but were also likely to visit again, he said.

Mr Karamaloudis said a large proportion of visitors were from outside Bendigo, but many residents also visited the buildings on show.

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54 per cent of visitors to Open House Bendigo came from Bendigo itself, while 37 per cent came from Melbourne.

59 per cent of visitors stayed one of more nights in Bendigo, while 92 per cent said they were more likely to visit the city again.

"There was a large contingent of Bendigonians that came out to see the many historical buildings that were open," Mr Karamaloudis said.

"The parts of the buildings that were exposed during the tours are typically areas that you don't normally get to see."

The program for Open House Bendigo 2019 is yet to be finalised, but Ms Bennett said it would go back to some key buildings, but would also include new sites.

The event's panel of talks and conversations will also expand in 2019.

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