Self-storage firm Big Yellow Group Plc sees a 15-20 percent earnings growth for the year on continued demand for its stores from London and the South East, its chief executive said.

The British company, which helps people and businesses store merchandise, goods, equipment and furniture while moving homes and offices, also expects higher demand from companies, particularly small and medium enterprises, looking to downsize their operations amid an economic downturn.

Within the UK, London and the South East economy is behaving better than certain other regional parts of the country, which are more dependent on public spending, Chief Executive James Gibson told Reuters.

In this kind of suboptimal environment...we can deliver 3-4 percent occupancy growth and double-digit, perhaps 15-20 pct, earnings growth.

The company's domestic customers, including individuals moving homes, occupy 65 percent of its storage space, while businesses take the rest. It delivers 88 percent of sales from London and the South East.

Builders and developers focussed on the South East of England are benefiting from stronger demand than the rest of the country. Sales in areas such as Wales, the North West, Yorkshire and Humberside can take up to twice as long as those in London, according to data from property analytics firm Hometrack.

Big Yellow said the seasonal decline in occupancy levels in the third quarter would be lower than a year ago.

The period since October 1 is historically our weakest quarter as students move out and the like, but this year, seven weeks into the quarter we are down 15,000 square feet, which compares to a 42,000 sq ft decline in 2010, Gibson said.

Earlier in the day, the company, which competes with Safestore and Lok'n Store , posted a 20 percent rise in first-half adjusted pretax profit at 11.6 million pounds as store sales increased 7 percent to 32 million pounds.

Big Yellow shares, which have lost more than a third of their value over the last six months, were up 4.6 percent at 228 pence at 1105 GMT on the London Stock Exchange.

(Reporting by Tresa Sherin Morera in Bangalore; Editing by Don Sebastian)