Washington (CNN) Democrats in the Senate are raising concerns about the potential cost of President Donald Trump's border wall -- releasing documents Tuesday from the Department of Homeland Security that explain what the administration is planning.

The first $1 billion of wall money requested by the administration would cover 48 miles of new wall, as CNN has previously reported

Democrats from the Senate Homeland and Governmental Affairs Committee released a slide presentation from Customs and Border Protection explaining the rationale for the initial wall segments.

The money requested by the administration for 2017, which Congress would have to appropriate, would fund 14 miles of new border wall in San Diego, 28 miles of new levee wall barriers and six miles of new border wall in the Rio Grande Valley region and 14 miles of replacement fencing in San Diego.

The documents also say that the agency will begin planning for 47 more miles in the Rio Grande City region or El Paso area of Texas.

The focus on the Rio Grande Valley region is because it is the "priority" for Border Patrol, CBP's briefing says. The decision to focus there is also supported by terrain and environmental analyses, a nearby urban center and roads, "short adversary vanishing times" and sector priorities.

The 14 new miles in San Diego, paired with an additional 14 miles of replacement barrier, were selected because the building would be ready to go as the federal government owns the land, as well as the analyses that supported the Rio Grande segment.

In 2018, the agency would hope to build 24 more miles of border barrier in either the Tucson sector in Arizona or El Paso region in Texas. Tuscon would be appealing to stop drug traffickers and "prevent armed conflicts between US-based rip crews and armed mules with the Sinaloa Cartel," the documents say. El Paso would improve existing barriers and build up where none exists now.

The documents also include diagrams of how DHS would structure "the enforcement zone," showing how in both urban and rural areas, any border wall would be set back from the border on the US side, leaving a buffer zone between the actual border of the country and where the wall is. That would include along the Rio Grande river, which in some places essentially forms the US-Mexico border.

As for what the wall will look like, the documents make clear that DHS is still moving forward with its prototype evaluation, pitting various potential wall options in San Diego to evaluate how the prototypes perform next to previous existing barriers. The region was also chosen because that area is federally owned and can be built on right away. Two sample photographs included in the presentation show a tall fence with slats so that agents can see through them.

After the presentation, the Democrats on the committee wrote in a report that they are concerned that the wall's cost has no "reliable estimate," and could stretch into the tens of billions.

The committee minority also raised the additional costs of acquiring land and maintaining the wall, which have not factored into the administration's budget request so far.

According to an email received by committee staff, maintaining the more than 650 miles of current border barrier currently costs $55 million per year, which comes out to $85,000 per mile.

The price tag of acquiring land could also substantially increase the overall budget. As CNN has previously reported , much of the land at the border is privately owned and legal costs to seize the land and pay the owners could be hefty.

The Democratic report says in one previous example, a Cameron County, Texas, landowner was offered $233,000 for 3.1 acres of land, but a three-year legal battle ended up raising the price for the government to $4.7 million instead.

Litigating such cases could also take a decade or more, the minority said.