Angry councillors and parents have objected to work starting on a school classroom before planning permission has been granted.
A planning application has been submitted for a portable classroom for seven years at the Vale Primary School, Beaconsfield Road, Langley Vale, Epsom.
But County Councillor Eber Kington said work has already started on site before planning consent has been granted by Surrey County Council.
Coun Kington said it is also not known how the school will deal with the additional traffic volumes. He said: "This situation is unacceptable.
"SCC sets a dangerous and morally unsound precedent to developers if SCC itself can ignore the planning process and start building on the basis of conversations amongst county officers, who get the go-ahead for work before planning permission if granted.
"Expanding Schools before putting in place in plans to deal with traffic congestion is a nonsense and a failure of their duty to local residents and the children attending the school.
"Deciding in July that a school needs to be expanded in September shows a failure of the school planning regime to understand the data on births which they have had in their possession for last few years."
Epsom borough councillor Liz Frost said: "The main reason for our protest is that the proper planning processes have not been followed."
She added: "Because there is no permission for building work, there are no conditions applied, and neighbours tell me that there was loud building work at 7am on Saturday - surely not acceptable.
"As to the plans themselves, when the school was first built, I understand that local residents were assured that the site was too small for there to be any expansion to it, therefore the proposals appear to be over-development of the site."
In a statement co-headteachers Carrie Christer and Claire Regnard said: "With the additional reception class starting in September 2014 all local children were able to gain a place, which is reassuring and comforting for parents in what can be a very stressful time.
"With this additional reception class the school will still be under its capacity of 210."
In response to questions from Coun Kington, SCC cabinet member for schools and learning Councillor Linda Kemeny said there was an urgent need to school places by September 2014.
She said: "The applicant in this instance has recognised the duty and obligations to Surrey parents and taken the difficult decision to submit their planning application, enabling local residents to register their views for consideration, whilst running in parallel the development of the scheme."