The company, whose UK operation is a joint venture between Yum! Brands and Whitbread Group , is planning a soft launch for the revamped sites, half of which are eat-in restaurants and the other half home delivery outlets.
All 12 are in the Uxbridge, Ealing and Wembley areas of London.
The outlets are being refurbished in line with Pizza Hut's latest store designs, which feature more modern and sophisticated decor than traditional Pizza Hut restaurants.
Items including goat's cheese bruschetta and nachos, which have not previously appeared on the Pizza Hut menu, will form a critical element of the food offering as the company attempts to broaden its appeal beyond pizza and pasta dishes.
The project forms part of Pizza Hut's UK investment programme, which involves the regular addition of new dishes, supported by marketing campaigns, to highlight the diversity of its menu.
"We trial products in locations around the world on an ongoing basis and for reasons of commercial confidentiality we neither confirm nor deny the specifics," said a Pizza Hut UK spokesman.
Other restaurant chains, including McDonald's, are undertaking similar initiatives to compete for the custom of increasingly health-conscious consumers.
Last month, Pizza Hut announced that it would open 75 restaurants this year to add to the 525 outlets already in operation.
More than 400 of these are full-service dine-in restaurants, while 116 are home delivery outlets. The expansion will create 3500 jobs this year.
Pizza Hut is the UK leader in the pizza diner market.
Brand health check, page 13.