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The Weald & Downland Open Air Museum first approached Boxall Sayer Construction Consultancy (BSCC) in April 1995 enquiring if they would be interested in acting as Quantity Surveyors for a major project within the grounds of the Museum. The Museum also indicated that assistance was required in preparing a bid to present to the Heritage Lottery Fund. BSCC had experience of the preparation, negotiation and assessment of bids for the Arts Council, Sport England, Heritage Lottery Fund and the Millennium Commission, and following discussions with the Museum's Director, was appointed to the role of cost consultants.

The Museum chose top designers Edward Cullinan Architects and Buro Happold structural and services engineers.

The initial task was to prepare a budget cost for the proposed structure to house a building conservation centre and open access museum. The proposal was to construct an innovative and non-traditional structure from greenwood timbers in the form of a gridshell at the upper level with a traditional blockwork constructed basement at the lower level.

The first budget estimate could only be assessed on the basis of cost per m2 of gross internal floor area - based on the Architect's basic sketch drawings as insufficient information was available for detailed appraisals.

As the design developed further information became available which enabled BSCC to transform the estimate into a more detailed cost plan. However, due to the unique nature of the proposed building, it was impossible to utilise information from current tenders or building cost information. In order that a realistic cost plan was produced in was essential that BSCC liaised very closely with the Museum and design team to establish the form of construction and likely quantities of timber lathing and connectors which would form the gridshell structure. With the aid of computer designs, three dimension diagrammatical building shapes were produced which enabled BSCC to monitor quantities and keep the Museum and design team aware of cost implications.

At this stage BSCC was also asked to act as Planning Supervisors under the CDM Regulations and to provide disabled Access Consultancy services to the scheme. It was particularly important that, due to the unusual shape of the building and the difficult environment, risk hazards and access requirements were carefully identified.

Buro Happold produced a scaled model of the structure which was used to estimate the total length of timber laths required to form the gridshell. As virtually all wall and roof surfaces to the gridshell in both vertical and horizontal planes were curved the use of scale rules to measure the various elements was eliminated. Subsequently BSCC's estimate for this element of the gridshell was proved by computer calculations to be remarkably accurate.

An interesting aspect of the design process was that initially the intention was that the flat grid of lathing would be laid out at ground level and raised into position to form the requisite "rolling" shape by means of cranes. Subsequently, and after consultation with specialist scaffolders, it was decided that an alternative method to be considered would be to lay the grid of lathing on a platform approximately 7 m above ground and then lower it into position thus taking advantage of gravity rather than competing against it. The latter method is being adopted on site.

Due to the complexity of construction and introduction of specialist craftsmen with new ideas BSCC attended numerous design meetings in order that they remained conversant with the development so that they could monitor estimated costs. This was an essential exercise as the final estimate not only formed the basis of the Lottery bid but it also kept the Museum informed of their obligations regarding matched funding.

During the design period it was decided that tenders should be obtained from three specialist carpentry companies, the successful tenderer would initially join the design team before becoming the named specialist carpenter sub-contractor who would construct the gridshell.

Tenders were obtained and the successful carpenter became an integral member of the design team and was able to provide invaluable advice to the team and cost information to BSCC

In due course a final cost plan was produced and a successful Lottery bid was made. On the advice of HLF the Museum extended the appointment of BSCC to also act as Project Managers to monitor progress and liaise with the Museum, HLF and members of the design team.

Due to the unique form of construction additional information is becoming available as the gridshell is being formed and BSCC team members are constantly visiting site to keep abreast of any developments which may have a financial effect. Therefore both HLF and the Museum are being kept fully informed of the anticipated final cost. This is particularly important to the Museum in order that they can ensure that matching funding is available

Currently costs of the building are in the region of £1,100 per m2 which is comparable to a similar sized building utilising traditional designs and construction materials. The cost is higher than originally expected but this can generally be explained as being due to costs arising as the scheme developed and the use of advanced building technology skills. BSCC would expect that the costs of future buildings adopting gridshell technology to be less expensive as the "learning curve" costs will be substantially reduced.

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