A Norfolk independent school's bid to build a �1.1m extension to its sixth form has hit a towering obstacle - an ancient tree.

Norwich High School for Girls on Newmarket Road has had to radically alter the design of the planned building so that it does not interfere with the roots of a wellingtonia tree in the grounds.

The problem 'significantly affected' the design of the building, which had to be switched from traditional foundations to a raised design to preserve the air and water supply to the tree - also known as a giant sequoia.

It added time and cost to the project, which is now moving forward - despite a further delay because regular rain in recent weeks meant the developers were unable to put fireproof paint on its steel frame.

The school now hopes that the project, overseen by constructors JS Hay, will be completed by the end of February or the beginning of March, with the students using it immediately it is finished and an official opening lined up for the end of April.

Business manager Peter O'Connor said the building included an eye-catching 'drum' design, with wood cladding. But the drum did not originally exist in the plans, only emerging after the ancient tree intervened.

He said: 'We were forced into a redesign because there's a tree on site. The original design would have impacted on the wellingtonia roots.

'We had to redesign that, which has had a big impact on the shape and cost. We couldn't put down foundations. We had to put them on a raft system.'

The school had to organise a root radar scan, the digging of trial pits and then the ground had to be carefully dug up to avoid the roots. The soil was then protected from concrete contamination, under the watchful eye of a local arboriculturalist - tree expert - and a tree protection officer from Norwich City Council.

Mr O'Connor said there were also issues around protecting the original Lanchester House building, which is listed and in a conservation area.

Headmaster Jason Morrow said the centre would comprise a lecture theatre, cafe, common room, break-out study areas, an art studio, a sculpture garden and a careers centre.

He said: 'The additional space that the girls will have will be inspiring. They will have much more room to do independent work and projects.

'It's going to be a beautiful building. The wood cladding will look lovely, while there is a terrace on the second floor.'

The school has put together a working party of year 11 to 13 students, who are helping to come up with colour schemes, finishes and furniture choices for the inside of the centre.

Do you have a schools story? Call Steve Downes on 01263 513920 or email steve.downes@archant.co.uk.