Managing director Julian Winfield said the garden centre's daily operation will be unaffected by the work, which will double capacity to 400 (HW, 13 December) and is scheduled for completion in October.
Last year the restaurant contributed 12 per cent of the centre's £9m annual turnover.
Winfield said: "The additional space created will allow the introduction of features including a coffee bar, pizza oven and daily carvery. The kitchen, storage and other facilities will also increase in size."
Southampton-based HPW Partnership has designed the new restaurant. An extra 20 full- and part-time staff are to be hired.