Designers working with McDonald's on its store redesigns in the UK have expressed doubts over the chain's "McHealthy" strategy as a way to answer critics of its food and rebuild sales.

McDonald's is unveiling three interiors concepts in the UK to try to revive its flagging appeal among a broader cross-section of consumers after sales fell globally for the 14th straight month.

Mark Brown, design director at Soft Moss, who is working with McDonald's on its interiors revamps, says McDonald's should focus on its original values: "It's the place people go to for hamburgers, so it should stick to what it's good at and create the best burgers around."

Brown believes it spells trouble for chains when the food on offer confuses consumers. He said: "McDonald's, like other fast-food restaurants, is starting to diversify too much. It's selling children's food, chicken wraps and coffee as well as Big Macs. People will think, 'why would I buy a salad from a burger place?' Brands can only stretch so far."

Neil Clayton, a director at Adcock Clayton, and part of the team that is creating pilot interiors for McDonald's restaurants in Dunstable and Brighouse agrees. He said: "I don't know if the public will buy the McHealthy approach, because McDonald's is McDonald's. It's quick, simple and easy."

Clayton said it was vital to achieve congruence between brand perception and the food offer: "Rejuvenating the appeal of the McDonald's brand involves linking food, marketing and interiors. In trying to attract back lost customers, it's primarily about products, but the environments make a big difference."

Brown agrees environments are important. He says they need to appeal to as broad a cross-section of people as possible while catering for local demographics.

At the main Oxford McDonald's branch, which Soft Moss is redesigning, Brown says designers have focused on the different types of customer likely to patronise the restaurant - tourists, students and families. The plans include refectory-style seating for students and large circular tables for families, with room for pushchairs.

Future store redesigns will vary between locations, but everything will be imbued with a "McDonald's-ness", Brown says. Soft Moss's ideas for drive-through restaurants will cater for drop-in customers, with interiors featuring "informal spaces" to encourage everyone, from "lorry drivers to divorced dads with their kids", to feel comfortable and "as if they were sitting round a kitchen table", he says.