Just how long does it take to build a single-family house? It depends on who’s building it, and where it’s being built, according to a report from the U.S. Census Bureau.

The bureau’s 2017 Survey of Construction found that the average construction time was about 7 1/2 months, including one month from the time the project was authorized to when building actually began, an Eye on Housing report from the National Association of Home Builders said.

On average, houses that took the least amount of time to build  were those built for sale (6.9 months). People who built their own houses on their own property took the longest — 12.3 months on average — while homeowners who hired a contractor to build a house on land they owned saw the house completed in an average of 9 months.

Other tidbits from this year’s survey:

• Construction times varied substantially by region. In New England and the Mid-Atlantic, construction times were 10.4 months and 10.3 months respectively. In the South Atlantic region, the time was only 6.4 months. On the West coast, construction averaged 8.5 months.
• It took builders a minimum of two weeks to begin work after permits were issued, but the delay could be much longer. In the Mountain region, builders were relatively speedy and needed only 17 days to start work. In New England, the wait was an average of 36 days, while on the West coast it was 39 days.
• Builders in metro areas are speedier than those in rural areas. The difference between metro and non-metro completion times was in some cases significant. In the Pacific region, for example, single-family houses in metro areas took a little more than 8 months, on average, to complete while houses in non-metro areas took more than 14 months. But the spread in other areas was much smaller — in the South Atlantic region, for instance, the difference between metro and non-metro completion times was only a few weeks.

The numbers have been trending slowly upward since the Census Bureau began collecting the data 47 years ago. In 1971, builders zipped through a single-family house in an average of 4.8 months — just 4.4 months for houses built for sale, and 7.2 months for owner-built houses.

The surveys don’t directly explain why construction takes longer now than it used to, or why metro builders are faster than their country cousins. But other information collected by the Census Bureau may offer some clues. Houses, for example, are certainly bigger (a median of 2,426 square feet in 2017 compared to 1,525 in 1973). They also have more bathrooms (only 30,000 houses built in 2017 had 1 1/2 baths or less), and are probably somewhat harder to build as codes require more insulation and air sealing.