Throughout the history of the Bay Area Rapid Transit, there have been plans to extend service to other areas.
An 8.7 km (5.4 mi) extension of BART south past Fremont to the Warm Springs District in south Fremont, with an optional infill station at Irvington. The Warm Springs extension will bring BART south to the Santa Clara County line. It is a prerequisite for the extension to San Jose, which will continue BART's expansion beyond the county line.
Soil studies have started in this area as engineering continues. This extension received a green light from the federal government when the Federal Transit Administration issued a Record of Decision on October 24, 2006. The action enables BART to begin purchasing the necessary right-of-way for the project and receive state-administered federal funding to finance the project.
In January 2009, the San Francisco Chronicle reported that $91 million in funding had been diverted from a similar project to link the peninsula and East Bay with commuter rail in order to begin construction on the Warm Springs Extension. 
On February 10, 2009, BART requested bids for the first segment of the Warm Springs Extension, a subway under Fremont Central Park and Lake Elizabeth. The joint venture of Shimmick Construction Co. and Scanska USA Civil West California District Inc. won with a bid of $136 million which was 45 percent below the agencies' estimate of $249 million. On August 24th, 2009, BART issued a "Notice to Proceed" to the contractors with construction to begin in Fall 2009. The contract for the above ground elements of the extension will go to bid in late 2009 with construction to begin in 2010. BART plans to have the extension open in 2014.
The City of Fremont is making an effort to get together city redevelopment funds and other sources to fund the optional Irvington Station. In the economic downturn, there remains uncertainty whether that will succeed.
Since Santa Clara County is not among the member counties of the BART District (having opted out of the district at its inception, like neighboring San Mateo county), VTA will be responsible for building the extension within Santa Clara County. VTA allocated initial funds for constructing BART using the proceeds from a sales tax referendum which was passed by Santa Clara County voters in 2000. In December 2002, VTA purchased a freight railroad corridor from Union Pacific Railroad which will serve as much of the necessary right-of-way for both the Warm Springs and San Jose extensions for $80 million. In 2004, the Federal Transit Administration decided to wait to fund the project, citing worries that BART did not have enough money to operate the extension. In addition, the San Jose extension project received a "not recommended" rating from the Federal Transit Administration placing the federal portion of the funding in jeopardy because of concerns about operation and maintenance funding. To address these concerns and help secure federal funding, Santa Clara County voters were presented with 2008 Measure B, a 1/8-cent sales tax raise, in the 2008 presidential primary election. Projections by an independent consultant recommended by the Federal Transit Administration predicted that the 1/8-cent sales tax would more than cover operation and maintenance of the planned extension. In initial vote counting from election day and well into the counting of absentee and provisional ballots, the measure was too close to call but was on the side of failing. On November 17, the measure flipped from failing to passing in the vote count update, with the required 66.67% of voters approving the tax increase, the very minimum for the tax measure to pass. On November 21, the result was announced that Measure B passed with 66.78% voter approval.
The planned route will continue south from the Warm Springs station in Fremont. There are plans for one "optional" station at Calaveras Blvd/SR 237 in downtown Milpitas; this station is not currently planned to be built but remains an option as a future infill station. The next station in Milpitas will be at Montague Expressway, co-located with the existing VTA Montague light rail station. The end of the extended line will be in San Jose, where there will be an elevated station at Berryessa Road at the current site of the San Jose Flea Market. BART tail tracks would end at US 101 between the interchanges at Santa Clara and Julian Streets.
The original plan was for the extension to continue into downtown San Jose via subway. However, in February 2009, projections of lower-than-expected sales-tax receipts from the funding measures forced the VTA to scale back the extension, ending it at the Berryessa station and delaying tunneling under downtown San Jose to a future phase of construction. The originally-planned extension from Fremont to Santa Clara would cost $6.1 billion, but the VTA estimates an extension to Berryessa would only cost $2.1 billion.
The plans for the downtown subway start with a subway portal before crossing under US 101. The Alum Rock subway station would be on North 28th Street between Julian Avenue and Santa Clara Street. The Downtown San Jose station would be underneath Santa Clara Street spanning the block from 3rd Street to Market Street. (The Downtown San Jose station was combined in 2005 from earlier plans for separate subway stations at Civic Plaza/San Jose State University and Market Street.) The Diridon/Arena station would be between the San Jose Arena and the Diridon train station, which currently serves Amtrak, Caltrain, ACE and VTA light rail. The BART subway would then turn north, following the Caltrain route, and exit to the surface at a subway portal after crossing under I-880. The Santa Clara BART station would be co-located at the existing Santa Clara Caltrain station. Separate construction plans by San Jose International Airport would bring a people-mover train to the Santa Clara BART/Caltrain/ACE/Amtrak station.
For the subway segment in San Jose, VTA plans to use a tunnel boring machine for most of the length in order to reduce disruptions to downtown during construction. Only the station locations would have cut and cover construction. This is different from how the BART subways were built in San Francisco and Oakland, which used the cut and cover method. The construction of the cut and cover stations in downtown San Jose would still cause major (albeit temporary) disruption, including closing several blocks of Santa Clara St and severing the VTA light rail line at Santa Clara St.
The Oakland Airport Connector is planned as a people mover that would directly connect from BART and Amtrak at the Coliseum station to the terminal buildings at Oakland International Airport. This connection would physically resemble other off-airport connections between airport people movers and transit trains, such as at New York JFK Airport and Newark Liberty International Airport. However, unlike the AirTrain, the Oakland Airport Connector would be operated by BART, integrated into the BART fare system with standard BART ticket gates located at the entrance to the station at the Airport end of the people mover. The OAC is currently awaiting public review of the final ridership analysis, after which the BART board will act to approve the last portion of federal funding needed for construction.
The eBART expansion plan calls for diesel multiple unit (DMU) train service to be implemented from the existing Pittsburg/Bay Point station. The first phase of the expansion will proceed east along the Highway 4 corridor to the town of Antioch. The plan includes an option for a station at Railroad Avenue in Pittsburg. Future expansions in this direction could connect the eBART service to Oakley, Brentwood, Byron, and beyond to Tracy and Stockton. The DMU system was chosen as a less-expensive alternative to the existing third-rail BART design. Funding for this expansion was approved in April 2009.
This extension of either conventional BART or DMU BART service would go from Dublin/Pleasanton station east to Livermore and over the Altamont Pass into Tracy and the Central Valley along I-580. It could possibly also go north through Dublin, San Ramon, Danville, and Alamo to the existing Walnut Creek station via the I-680 corridor.
A Program EIR has been started and is in the planning stages. BART has set up a special website to track the Bart to Livermore extension plans. Currently, a number of different alignments are being considered. This comes after a petition to extend BART to Livermore was circulated by Linda Jeffery Sailors, the former mayor of Dublin, California.
The extension of conventional BART rail to Tracy is considered unlikely, as San Joaquin County, in which Tracy is located, is not part of the three district counties and does not pay into the regional BART tax. The extension of third-rail BART, which would require exclusive and grade-separated rights-of-way over such a long distance, would be substantially more expensive. With conventional rail, existing trackage can be used, and incremental upgrades (such as grade separations at selected intersections, overhead electrification, signaling improvements, utilities relocation, etc.) are possible as funding dollars become available, but choosing BART would require a full build-out of the system initially, along with comprehensive funding.
An existing diesel commuter rail line, the Altamont Commuter Express (ACE) currently operates on much of this route. A free shuttle transfers passengers between the ACE Pleasanton station and the BART Dublin/Pleasanton station, linking the two systems.
A corridor study of extending the service north from the Richmond Station is underway with numerous options being studied. One would create commuter rail service utilizing lightweight diesel multiple units (DMU) to operate on existing or new tracks. In order to operate on existing tracks with freight service, however, heavier-weight DMU vehicles adhering to Federal Railroad Administration regulations would need to be used. This option is known as wBART. A second option would create a commuter rail service running from the BART terminus along the Amtrak line to Hercules and possibly Fairfield and Vacaville in Solano County, similar to the Caltrain or ACE train services. Yet another option would extend conventional BART to a North Richmond station near the Richmond train yard at 13th Street/Rumrill Avenue and Market Street, then continue along the existing Union Pacific rail line and the Richmond Parkway expressway to Interstate 80. The service would have a Hilltop station and then continue along I-80 to State Route 4 in Hercules, near Hercules Transit Center. Service would continue along I-80 through Vallejo until the I-505 interchange in Vacaville. Finally, a proposed option would stretch BART westward across the Richmond-San Rafael Bridge into central Marin County.
BART planners have studied and/or planned infill stations for at least three sites within the system. Infill stations are stations constructed on existing line segments between two existing stations. Construction costs for a planned 30th Street Mission station in San Francisco, between the existing 24th Street Mission and Glen Park stations, are estimated at approximately $500 million. A proposal for a Jack London Square station in Oakland was rejected as being incompatible with existing track geometry. A one-station stub line to Jack London Square at the foot of Broadway and the use of other transit modes also were studied.
The new infill West Dublin/Pleasanton station is currently under construction in the median of I-580, just west of the I-680 interchange between the Castro Valley and Dublin/Pleasanton stations. Construction estimates for this station are $100 million, with funding coming from a unique public-private partnership and transit-oriented development (TOD) project on adjacent BART-owned property. Construction on the station began in October 2006, and was slated for completion in 2009, but faulty welding has potentially delayed opening until 2011.