A state law that took effect this year closes a loophole that let people hide the sales price of real estate by withholding the amount of transfer tax from public documents.

The new law requires the documentary transfer tax to be shown — without exception — on the first page of the deed.

In the past, some rich people and celebrities used the decades-old loophole to prevent the public from finding out how much they paid for a home. The practice was frustrating for nosy neighbors and appraisers looking for comparable sales. It also made extra paperwork for county recorders. The information is now readily accessible to any member of the public.

Every California county, and many cities, charge this tax when real property changes hands. Because the tax is a percentage of the sales price, if you know the tax and location you can figure out the sales price if it’s not disclosed elsewhere, such as in a Multiple Listing Service.

Although state law already required every document subject to the tax to show the amount of the tax on the first page, before this year, the party submitting a document could ask that the tax be shown on a separate piece of paper that was not publicly disclosed.

AB1888 removed the secret-filing option. The bill was sponsored by the Appraisal Institute and authored by Assemblyman Phil Ting, D-San Francisco, formerly San Francisco’s assessor-recorder. It passed the Legislature unanimously.

Buyers can still hide their identity by purchasing property in the name of a Limited Liability Co. or other business entity.

It’s hard to know how many people had been trying to hide the transfer tax. Napa County Recorder John Tuteur estimated that 2 to 7 percent of recorded documents had the tax hidden. “It used to be just the big fancy guys, vineyards and shopping centers,” that asked for it, he said. But requests “had been increasing before Phil Ting pulled the plug.”

Alison Teeman, an appraiser in the East Bay, said that more than 90 percent of deeds include the transfer tax, but it was often missing on high-priced homes in certain neighborhoods such as the Happy Valley area of Contra Costa County. “When Buster Posey buys a house, he doesn’t want people to know where it is or how much he paid,” she said.

Brian Grey, who appraises homes in San Francisco and San Mateo County, used to see missing transfer taxes in places such as Woodside, Atherton and Hillsborough. “The upper-end properties are the hardest to appraise. They are almost always unique,” he said. Withholding the transfer tax made his job that much harder.

Before this year, it was possible to get a hidden transfer tax in some counties by filing a California Public Records Act request, but getting a response could take weeks or months. Sales amounts could also be deduced after a home was reassessed for property taxes, but that also could lag the sale by many months.

For most properties, sales prices are readily available because Multiple Listing Services require agents who list properties on the MLS to provide them. But exceptions were made. “If there is a specific request for it not to be disclosed, we would take that into consideration,” said Walt Bakowski, chief executive of the San Francisco Association of Realtors, which operates the city’s MLS. He said such requests were rare but typically honored.

Still, a growing percentage of homes in the Bay Area are being sold without being advertised on an MLS. These so-called pocket listings — often sold by agent word-of-mouth — accounted for an estimated 24 percent of the homes sold in San Mateo County last year and 20 percent in Santa Clara County, according to MLSListings.

In some of these transactions, the Realtor might provide the sales price to the MLS after the sale, to improve comparables and to get publicly acknowledged for the sale. But in many cases, the sales price is never entered into the MLS and the transfer tax is the only way of deducing it.

Real estate professionals can now get this information in private databases that scour recorders’ offices for public documents. Individuals can get it by calling or visiting a recorder’s office.