A joint district and county council group is asking the company to help fund a bypass on the A12 and dual the local railway line.
The government will make the decision on whether the power station is built.
EDF said public consultation on its initial plans would begin later this year.
Councillor Guy McGregor, chairman of the Suffolk County Council and Suffolk Coastal District Council Joint Local Authorities Group (JLAG), said: "The actual cost of the power station is £2bn, so £100m out of £2bn is almost petty cash.
"There's no way you can build that power station using road access without building a dual carriageway - the whole of east Suffolk would come to a standstill.
"Local communities can rest assured that we are already working together to mitigate the environmental impact of any future development at Sizewell while maximising the economic and social benefits created for our communities."
The councils would like an A12 bypass between Wickham Market and the Aldeburgh junction which would take the road around the villages of Little Glemham, Marlesford, Stratford St Andrew and Farnham.
JLAG estimates it would cost about £30m and said its other priorities would be for the dualling of the East Suffolk single line railway, the reopening of the track to Leiston and Aldeburgh and support for schools and colleges with training opportunities.
A spokesperson for EDF, which wants to build Sizewell C by 2025, said: "In preparation, we are currently working on a number of investigative studies into all the environmental, transport, infrastructure and socio-economic elements which will inform our plans.
"The project is at a very early stage and we plan to launch our first stage of public consultation on our initial proposals later this year.
"We look forward to hearing the views of local communities."
The energy firm said no figure had been revealed for the total cost of building Sizewell C yet.
Energy company EDF should spend £100m on improving infrastructure if a new nuclear power station is built in Suffolk, councillors have said.