The department store's London flagship on Oxford Street will dedicate 60,000 square feet to handbags and other accessories by 2018, within a single space containing more than 70,000 items on any given day.
The grand plan is a tripling of the current space "into the ultimate global destination for luxury accessories", the store said.
Designed in collaboration with architect David Chipperfield, the area will "bring greater meaning to each purchase" and "reinforce customers' appreciation of each product as a true investment rather than just a commodity".
The space will be without any internal dividing walls and will open in three main phases over the next two years, with completion expected for late spring 2018.
The store's Duke Street facade is also being given a major facelift, with a triple-height entrance and design to maximise its proximity to the Bond Street Crossrail station due to open in December 2018.
Selfridges said the plan was created around the never-realised ambition of the store's founder Harry Gordon Selfridge to create a unified block within the Oxford Street, Orchard Street, Wigmore Street and Duke Street quadrant.
The refurbishment project is costing "in excess of £300m", the store said.
A spokesman said: "We are thrilled to be unveiling the first phase of our Accessories Hall, which represents the first major externally visible milestone in the ambitious transformation of the Duke Street side of our store.
"It's the first time in 40 years that such dramatic architectural changes designed to challenge how our customers experience this part of our store, on both sides of the facade, are being undertaken."
Last month Selfridges reported a drop in full-year profit following the company-wide investment programme that bit into the balance sheet.
The retailer said that operating profit dropped 1.9% to £152m in the 12 months to 31 January, compared with a year earlier, despite seeing a 5% jump in gross sales to £1.4bn - a record for the company.
The firm stressed that it was two years into its "ambitious" revamp of its London and online store, which marked an "unprecedented level of investment" for the retailer.
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