* SNB leaves rates unchanged as expected in poll
* Central bank gives banks relief on negative rates
* SNB cuts GDP growth, inflation forecasts (Rewrites, adding detail, reaction)
By John Revill
ZURICH, Sept 19 (Reuters) - The Swiss National Bank expects to stick to its ultra-loose monetary stance for the long haul after it kept its policy on hold on Thursday and handed banks hit by its negative interest rates extra breathing space.
The SNB left its main policy rate unchanged at -0.75%, bucking moves by the European Central Bank and the U.S. Federal Reserve, which have cut rates to tackle the slowing global economy.
It maintained the rate it charges on commercial banks' sight deposits at -0.75%, as expected in a Reuters poll.
But the SNB increased the exemption threshold above which commercial banks who park their money with the central bank have to pay negative interest.
The new rules, which take effect on Nov. 1 and can be adjusted monthly, raise the amount banks can store with the SNB before negative rates apply to 25 times their minimum reserves from 20 times now.
The move was welcomed by the Swiss Bankers Association, which has said the charges disadvantage the financial sector and damage the Swiss economy.
Banks were paying out 2 billion Swiss francs ($2.02 billion)per year in charges, the SBA said.
"It is good that the rates have not been reduced further and the expansion of the exemption seems to be a good thing," said SBA spokesman Serge Steiner.
"But the burden with negative rates is still a burden for the banks. It is a problem for the pension funds, for the retail clients and the Swiss economy on the whole."
Credit Suisse declined comment on the change, while UBS did not immediately reply to a request for comment.
The extension is designed to support banks who have complained about negative rates and dissuade them from passing charges on to customers.
It also recognises the increase in sight deposits as a result of the SNB's massive currency interventions to weaken the franc. Deposits have expanded to 471 billion at the end of June from 56 billion francs in July 2011 -- 27 times their combined minimum reserve requirements.
The SNB credits the sight deposits of Swiss banks with newly created francs in exchange for foreign currency.
"This adjustment to the calculation basis takes account of the fact that the low interest rate environment around the world has recently become more entrenched and could persist for some time yet," the SNB said in a statement.
The adjustment means negative SNB rates are likely to stay longer than previously anticipated, said Charlotte de Montpellier, an economist at ING.
"We are a long way from normality and positive interest rates. Several years at least," she said, noting inflation forecasts remain very low.
The SNB called the franc "highly valued", sticking to its description of the currency whose strength weighs on Swiss exporters by making products more expensive in the euro zone.
The SNB cut its Swiss GDP forecast and its expectation of inflation to reflect the weakening global economy. ($1 = 0.9910 Swiss francs) (Reporting by John Revill; Editing by Michael Shields and Emelia Sithole-Matarise)