The council planning committee deferred "for negotiations with the applicant on planning contributions and further assessment of condition four relating to goods to be sold".
The extra 3,000sq m would create hundreds of jobs, said Wyevale.
But Alex Turco of Burnham and Highbridge Chamber of Trade warned the move could cost Brent Knoll's economy £4 million if given permission.
The town council has lodged several objections including concerns over the loss of town centre jobs to the garden centre and the negative effect on regeneration.
Town planners have also said the "range of goods is far above the needs for most garden centres already" and "complaints about this have had little impact".
Sanders plans on increasing its range of goods for sale to incorporate items such as camping equipment and seasonal goods - impacting town centre stores, according to the town council and chamber of trade.