What is the best way to have a fitted kitchen designed, supplied and installed as part of a house extension or refurbishment project? I am often asked this as there are various approaches.

Island kitchen, Dormans Park, nr East Grinstead Underwood Kitchens
One approach is to have the fitted kitchen fully designed and specified by the architect and installed by the builder as part of the contract works. This involves the architect specifying all of the components including the base and wall units, countertops, splashbacks, sinks, taps and equipment. Both the architect and the builder are paid for their services with regard to the kitchen.
Another approach is for the client to purchase the kitchen components outside the contract (either designed and specified by the architect or by a specialist) and provide them ‘free issue’ to the contractor who installs the kitchen as part of the contract works.

L-shaped kitchen, Forest Row Homebase KitchensHomebase Kitchens
Yet another approach is to have the fitted kitchen designed, supplied and installed by a specialist outside the contract after the extension or refurbishment work is finished. This avoids paying either the architect or the builder for any services with regard to the kitchen but has the disadvantage of pushing back the date when the kitchen is operational.
I have evolved an intermediate approach whereby the standard form of building contract is amended with a clause added requiring the builder to permit access during the final weeks of the contract (it is his site so he can otherwise deny it) by a specialist directly engaged by the employer under a separate contract. Subject to negotiation this approach can allow the client to procure a specialist kitchen design, supply and install ‘package’. The builder provides a ‘serviced shell’ for the kitchen (to suit the specialist’s design) comprising the finished floor, walls and ceiling plus capped-off services in the shape of hot and cold water supplies, waste, extract ventilation, gas, power and lighting. The specialist then installs the kitchen into this ‘serviced shell’. Finally the builder makes the final connections for the services and all fitted kitchen appliances and includes the kitchen in the project GasSafe and electrical certification.
Kitchen design is a personal thing for all clients in terms of the style and layout of the kitchen, the type and amount of equipment and the expense which can vary by up to a factor of ten. I think it is best for the architect to simply set the design parameters in terms of the location and general layout (such as an L-shape, galley, island, peninsula etc) and then allow the client to develop and finalise the design, supply and installation with a specialist.