Housing price declines are gaining speed while inventory levels are rising, according to the real estate data provider Altos Research. The Altos 10-city composite price index decreased last month, and all 26 markets the firm tracks saw declines. The average national house price fell to $469,9100 in August, down about 1% from July and at the lowest level ever recorded by Altos. In June, house prices declined by 0.63%. “Where there had been blips of increased activity and asking price during the [second] quarter of 2010, those have been replaced by measurable decreases in price, as well as a steady increase in inventory,” according to Altos. “Nationally, week-over-week drops in price are gathering speed, making it clear that the small gains seen earlier in the year were generated by momentary external forces rather than an overall increase in consumer confidence.” Asking-price declines in August were steepest in Phoenix at 5.1% and Miami at 3.11%, with notable decreases in Dallas (2.4%), Boston (2.21%), and Tampa-St. Petersburg (2.2%), Altos said. And the firm expects further declines in the fall. Inventory fell most significantly during the month in Austin with an 8% decline from July, while Houston’s inventory of homes rose almost 5%. Write to Jason Philyaw.