The new building is being built next to the original garden centre and will include a 500-seat restaurant, the largest of any Haskins centre. The new garden centre will be 10 per cent bigger, with its retail area 20 per cent bigger.
More of the plant area will be under cover and a system of ventilation sashes is being installed in the roof.
Haskins chief executive Julian Winfield said the development was well on the way to completion.
"We have been very clear that we have done it for one reason only and that is to maximise our long-term commercial opportunities and reap the rewards as we have done since 1993 and 1995 at Ferndown and West End," he said.
He added that the company was still looking for a fifth site. "We are always on the look out for new sites, but funding is an issue and we don't want to take any risks."
Haskins opened a new restaurant at its West End Centre in Southampton last month. Winfield said £4m had been invested at the centre, which is trading better than budget.
Chairman Warren Haskins added that the restaurants were the most profitable side of the business. "It is going extremely well and represents about 20 per cent of our business," he said.
"But it's also the most complicated part of the business - the most difficult to run and to manage."