The Trump administration wants the first $1 billion of border wall funding to cover 62 miles -- including replacing some existing fencing along the southern border.

The $999 million requested by the White House in its budget supplement for just defense and border security spending would cover just 48 miles of new wall, according to justification documents from the Department of Homeland Security obtained by CNN.

The documents describe exactly where the administration hopes to put its first installment of the border wall, as it described its modest 2017 funding ask.

The money will 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 fencing would likely include concrete elements, a source familiar with the plans told CNN.

President Donald Trump made building a border along the entire US Southern border a centerpiece of his presidential campaign, and repeatedly pledged Mexico would pay for it, but the initial ask for funds to start building has been modest. The administration requested $1 billion in its supplemental for 2017, and another $2.6 billion for border infrastructure and technology in 2018.

Estimates for a full wall along the Southern border have ranged from $12 billion to more than $20 billion.

The budget request documents also make clear that the Trump administration is looking to ramp up border security elsewhere, specifying millions to bolster detention facilities and legal support for building the wall.

The documents also claim that Trump's executive orders on immigration enforcement are already having an effect, saying arrests are up 50%, charges are up 40% and requests to detain arrested individuals who are deportable are up 80%. When asked, ICE did not have information on what the administration is using as a baseline.

Wall or fence?

Although on the campaign trail, Trump described building a tall, concrete wall, many career officials within DHS recommended see-through fencing as a more secure option, as CNN previously reported, in part to have visibility on what's happening on the other side.

The federal government is soliciting contracting bids for two types of construction according to the bid website -- "one focused on concrete designs, and one focused on other designs."

W. Ralph Basham, a former Customs and Border Protection commissioner under the Bush administration, said instead of paying for a concrete border wall the government should strategically fund increases in surveillance technology and the construction of additional fencing based upon recommendations made by DHS.

"What does border patrol need to get the job done?" Basham said. "A lot of people in this country think that this wall is a good idea, but when you really break it down, it may not be the best way of spending the taxpayer dollar."

Homeland Security Secretary John Kelly testified to Congress in February that some border patrol agents made him aware of their concerns about a solid, concrete wall as opposed to see-through fencing.

Experts also said at the same House Homeland Security Committee hearing that a border fence could have advantages over a concrete wall, because agents could more easily see across the border.

Kelly said the wall will take time to construct.

"We're not going to be able to build a wall everywhere all at once," he said.

But he didn't give a clear timeline or cost estimates.

"I'd like to see we'd be well underway in two years," Kelly said of the wall. "This is going to take some time, but there's places I think we can right away get at this problem."