Seventy of its 163 food stores will be refurbished through the investment, which will also improve management processes for financial and stock issues.
Two new stores open in Eccleshall, Staffordshire, and Jacksdale, Nottinghamshire, in the spring.
More than £9m has been spent on store development this year, including two new supermarkets in Overseal, near Swadlincote, and Osmaston Road, Derby.
Post offices have been introduced into stores in Corby, Northamptonshire, and Sheldon, Birmingham. The refurbishment of the eight former Metro Group outlets bought in 2004 has been completed.
The society posted total sales of £359m in the half-year to 12 August, down from £365m last year. Pre-tax profit rose 56%, from £16.5m to £25.7m.
Like-for-like sales in the retail division rose 2% to £241m with supermarkets also performing well.