It will be carried out in two phases and will include a new two storey design and technology and science block, which will link to the existing building.
A new sports hall is being built which will house four badminton courts, together with an adjoining classroom and changing rooms.
An energy centre is being constructed, and external work will include a new school entrance, hard play areas, car park, bus parking area and games courts.
Phase two will comprise the refurbishment of various areas within the existing buildings together with re-modelling of playing fields.
The contract for the work was awarded to Andrew Scott Ltd after a tender process on the South West Wales Regional Contractors Framework.
It is expected that phase one will be completed before the 2015 summer holidays which will allow the school to occupy the new buildings from the following September.
The investment at Ysgol Maes y Gwendraeth is being made through Carmarthenshire County Council’s Modernising Education Programme.
It follows the merger of Maes yr Yrfa and Gwendraeth Valley schools as part of the Transformation of Education in the Dinefwr area.
Leader Cllr Kevin Madge and education executive board member Cllr Keith Davies were part of a group that visited the site to cut the first sod.
Cllr Madge said: “We’re really pleased to see the start of work on this school which will accommodate 1,000 pupils and provide them and the staff with the top quality learning environment they all deserve.”
Cllr Davies added: “This is what our Modernising Education Programme is all about, providing first class accommodation and facilities for all our children in Carmarthenshire.”
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