"vendor told me should be no problem to get planning permission to extend"
don't believe a word the vendor says, they will tell you anything to sell it
1. look at any neighbouring houses, has anyone else extended similar to what you want to do ? this would set a good precedent when talking to the planning department
2. as mentioned above talk informally with the planning department
2. check the local authority website for recent planning decisions in the area, or hire an architect on an hourly rate to do this for you and to make an assessment on the feasibility of planning, although they can't guarantee you will be successful
3. how large is the extension ? subject to certain conditions you could build up to 40sqm extension to the rear WITHOUT planning permission. again if you don't feel confident checking this, an architect could do it for you as a feasibility study for a fixed sum
guidance here > http://www.environ.ie/en/Publications/DevelopmentandHousing/Planning/FileDownLoad,1586,en.pdf