The additional space will provide accommodation for Police Scotland, along with shared office space for council and police colleagues to work alongside each other.
The two-storey extension will join the existing Aberdeenshire Council base, on St Peter Street, and share its facilities.
The plans have been developed by Robertson Construction, which has been appointed on a 'design and build' basis for the project.
The extension is being funded by Aberdeenshire Council and will be leased to Police Scotland upon completion, with the lower floor used by police officers and staff and the upper floor offering shared facilities. Permanent car parking space is also to be created on land currently used as overspill for the site.
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Work is expected to begin mid-May, with completion in January next year.
Chair of Aberdeenshire Council's Buchan Area Committee, Cllr Norman Smith, said: "It is pleasing to see the project reach this stage and I look forward to work beginning on site soon.
"The extension provides a great opportunity for even closer working with our Police Scotland colleagues and I am sure the people of Peterhead will welcome the increased presence at Buchan House."
Police Scotland already has a presence at the Buchan House Service Point and the extension will provide space for additional employees and services.
Chief Superintendent Campbell Thomson said: "This move is an exciting opportunity for Police Scotland to fully integrate services with Aberdeenshire Council, working collaboratively in our partnership approach to addressing local issues within local communities in Peterhead and indeed the Buchan Corner."
Gordon Milne, managing director for Robertson Eastern, said: "Over the past several years, Robertson has developed a close relationship with Aberdeenshire Council and this latest project cements our reputation as specialists in delivering bespoke builds for clients.
"As the original developer of Buchan House, we're excited to be part of the extension project that will bring much-needed additional facilities and space for Aberdeenshire Council and Police Scotland."
Buchan House will continue to operate as usual throughout the majority of the works and any temporary arrangements will be communicated in advance.
