The Duchess of Northumberland was today ''incredibly excited'' to have been given permission to build a £3 million tree-top haven behind Alnwick Castle.

Dubbed TreeTopia, it is a wooden tree house complex as big as two Olympic-size swimming pools that will be built near her husband's ancestral home.

The development will be part of grander plans - The Alnwick Garden - to transform the once-derelict walled garden behind the castle.

The first stage of the garden project featuring 12 acres of cascading waterfalls, rushing rills and spouting fountains at a cost of £10 million, was opened by the Prince of Wales earlier this month.

Alnwick District Council granted planning permission for the grandiose plan late last night. The Duchess, Jane Percy, today said: ''We are all incredibly excited about this project and delighted that planning permission has now been granted.'' TreeTopia is part of the Duchess's master plan for part of the Duke's extensive land which she originally started as a ''pet project''. The development has since snowballed into a major tourist attraction, drawing crowds far in excess of visitor number predictions. TreeTopia will be seen as the icing on the cake and was billed as a place for visitors to ''learn, relax, play and 'lose themselves'''. The tree house will involve a complex of inter-linking wooden buildings featuring suspended catwalks, rope and cable bridges and zip slides more than 60ft above ground level. It will be built to the eastern edge of the Alnwick Garden in mature woodland. Schools and special interest groups will be able to bring learning to life in a series of resource areas. Activities ranging from gardening and cookery, art and school curriculum support will take place on site. The educational programme has already garnered the support of the Learning and Skills Council in recognition of the need to create new and innovative learning opportunities in the community. The Duchess said: ''As a resource area, TreeTopia will provide adults and children with an inspiring environment in which to learn. ''It will be a safe, fun area where children can play and explore, while the tree house and adventure area will provide a wonderful setting for their imaginations to run wild. ''In just our first year of opening, the Alnwick Garden will have attracted 275,000 visitors. ''I am confident that TreeTopia has the potential to significantly increase visitor numbers and continue to generate community involvement and increase trade and revenue for the town and region.'' The cost of the scheme is expected to be borne by fund-raising efforts already in motion for the larger Alnwick Garden development.