While it has been highly disruptive to the office sector in general and Piedmont's leasing pipeline specifically, we are fortunate that the vast majority of our customers are current on rent and building utilization continues to improve, now approaching an average of 25% across the entire portfolio, primarily led by a return to the workplace by our small to medium size tenants.
In addition, we are beginning to see medium-size space request in the 15,000 to 25,000 square foot range and starting to conduct tours with these tenants.
While we saw consistent upticks in tour activity across the portfolio as the first quarter progressed, a 30% increase actually from January to March, we still anticipate it will take two to three more quarters for Piedmont leasing volumes to approach pre-pandemic levels.
With the USD GDP expected to grow 6.2% in 2021, per Bloomberg's April economist survey, U.S. feels like it's getting back to a new normal.
The company completed approximately 678,000 square feet of leasing, with new tenant leasing accounting for approximately one-quarter of that activity.
We are finding that medium-size enterprises are having the greatest difficulty in reaching a conclusion and as such, these customers typically requiring 15,000 to 25,000 square feet are those most often requesting shorter-term renewals of two to three years.
As I noted in our last call, we continue to see the smaller user market, defined as those being less than 10,000 square feet remain rather resilient, almost approaching pre-pandemic levels of activity.
And in a large user segment, to find as those needing more than 50,000 square feet, we continue to see the companies who know their business well, use this market disruption as an opportunity to negotiate more favorable terms from their landlords.
I am pleased to share that one example of this phenomenon has resulted in an early seven-year renewal of Raytheon's approximately 440,000 square foot lease, comprising the entirety of our 225 and 235 Presidential Way assets in Boston.
Looking ahead, our only expiration of any significance over the next 18 months is the City of New York lease of approximately 313,000 square feet, that remains in holdover at 60 Broad Street.
Finally, following our playbook, on the State of New York's lease in 2019, we continue to work with DCAS on a potential 20 year extension at the building beyond this extension.
This is the first time that that tenant has executed renewal that provides a total of 10 years of remaining lease term at the buildings.
As the two assets represented by this renewal are now 100% leased to a single credit-worthy tenant with significant term, we believe that value has been maximizing the properties and we have a unique opportunity to realize that value which has been created.
Therefore, we began marketing our 225 and 235 Presidential Way properties for sale late in the first quarter and we received a good deal of interest.
On the sustainability front, out of 1,000 of participants in the U.S. ENERGY STAR program, Piedmont was recently named one of 70 company's designated the 2021 ENERGY STAR Partner of the Year and Piedmont is the only office REIT headquartered in the Southeast U.S. to receive this designation.
Approximately three-quarters of our portfolio is currently ENERGY STAR certified and we continue to make significant progress toward our goal of reducing the overall energy consumption by 20% at our properties over a 10-year period ending in 2026.
Our Atlanta Galleria properties representing over 2.1 million square feet of rentable space, were the first properties in our portfolio, as well as the first for all office post the Atlanta to receive this new rating and we're actively working to expand this program across our portfolio.
For the first-quarter 2021, our reported net income increased to $9.3 million, up 7.3% from the same period a year ago.
We also reported $0.48 per diluted share of core FFO up $0.01 over the first-quarter 2020.
AFFO was almost $39 million for the first quarter, well in excess of our current quarterly dividend level.
Accrual based rents increased on average approximately 7% while cash rents decreased approximately 2.8%.
However, excluding the strategic Raytheon renewal, cash and accrual rents for the remainder of the leasing activity rolled up 8% and 10.1% respectively.
Same-store net operating income increased almost 4% on a cash basis and was down slightly on an accrual basis.
The increase in cash basis, same-store NOI was primarily attributable to the burn off of significant abatements at 11.55 Perimeter Center West in Atlanta and Arlington Gateway in Washington D.C., along with income associated with WeWork's termination in Orlando.
These increases were partially offset by reductions and transient parking and retail revenues as a result of the lingering effects of the pandemic, and by 0.8% decrease in portfolio occupancy during the first quarter of this year.
We continue to believe that same-store NOI on both the cash and accrual basis will end the year positively between 3% and 5% and economic occupancy is also expected to improve with the burn off of over 400,000 square feet of abatement during the second quarter.
Our overall lease percentage is estimated to end the year around 87% to 88%, but this estimate is before any capital transactions.
Turning to the balance sheet, our average net debt to core EBITDA ratio improved during the first quarter of 2021 to 5.6 times.
And our debt to gross asset ratio at the end of the first quarter remained relatively flat, compared to 2020's year-end at approximately 34.9%.
Our tenant rent collections have returned to near pre-COVID levels at over 99% collections and we now only have approximately $3 million of previously deferred 2020 rents remaining to be paid during 2021.
We currently have approximately 90% of our $500 million line of credit available for strategic capital transactions, along with proceeds expected later this year from the sale of the two Presidential Way assets in Boston.
We plan to repay the only secured debt remaining on our balance sheet, a small $27 million mortgage, once the loan allows for prepayment without yield maintenance later this quarter.
We also plan to go to the public debt markets later this year to refinance a $300 million term loan that matures at the end of November.
At this time, I'd like to reaffirm our 2021 guidance in the range of $1.86 to $1.96 per diluted share for core FFO for the year.

Summary:
We also reported $0.48 per diluted share of core FFO up $0.01 over the first-quarter 2020.
At this time, I'd like to reaffirm our 2021 guidance in the range of $1.86 to $1.96 per diluted share for core FFO for the year.