Roccia, in Queen Street - already the UK’s biggest independent tile showroom - needs more space for its developing kitchen design portfolio.
Roccia spokesman Frank McLaughlin said: “This is our flagship store and its fitting that we’re making this investment on our 25th anniversary.
“It’s a really positive time for the company.”
In October, Roccia, which also has a showroom in Bolton, was named Medium Business of the Year at the BIBA awards.
The extension work began in February and is set to be completed in autumn.
Frank added: “We’ve been wanting to move into kitchens as well as bathrooms for some time, so this is the realisation of that.
“We have some real expertise here, in fact our kitchen designer, Daniel Porter, won the title of Kitchen Designer of the Year. He’s a real talisman.”
The company, which currently employs 72 people across the North West, is also running an apprenticeship scheme with Preston’s College.
Frank said: “We started that scheme earlier this year. We want to help younger people in the community get a start in what can be a lucrative trade, and it helps us make good contacts in the local area.”
Roccia’s investment in the local community also extends to its sponsorship of local sports teams.
The company sponsors Penwortham Cricket Club and Bolton Nomads.
Frank added: “It’s important for Roccia to be a big part of the community.”