The expansion to the south and west, taking up some of the existing car park, will also create a mezzanine level for staff and the other departments.
More non-food goods such as electrical items and clothes will be on sale and there will be an in-store cafe.
The total sales area will expand from 1,540 sq m to 3,380 sq m and the expansion will create 100 jobs.
In January, a spokesman for Tesco said the work would be carried out this year. But this week, corporate affairs manager Tony Fletcher said the expansion work was not in this year’s programme of construction work.
He said: “Our construction programme runs from February until February and the Oakham store is not in the current plan. The necessary paperwork wasn’t through in time to put it on this year’s construction programme because of the appeal to the Secretary of State.”
The plans had attracted criticism from people in the town and a petition with 1,500 signatures against the plans was presented to Rutland County Council before a decision was made.
Residents had objected to the size of the store and were worried a footpath to Penn Street would lead to nuisance behaviour.
At the last minute, Tesco changed its plans, which originally had included demolishing a bungalow in Penn Street to create more parking space. The footpath to Penn Street was kept.
After the plans were approved, the protesters wrote to the Secretary of State to try to get the decision overturned but were unsuccessful.
He added: “We are still committed to carrying out this expansion work in Oakham and we will be looking to carry out the work next year.”
He said more details on exactly when the extension would be built would be available in November, when the construction programme for next year is put together.