St. Mary’s County, Maryland

St Mary’s County, population 105,151 (2010), forms the tip of land where the Potomac River meets the Chesapeake Bay in southern Maryland. There are approximately 43,000 acres of prime soils scattered throughout the County, with the largest tracts of prime farmland in the southern part of the County. The County has remained primarily rural despite the fact that it lies only 40 miles southeast of Washington, DC. Like adjacent Charles and Calvert counties, St. Mary’s County included a transfer of development rights section in its zoning ordinance in about 1990 in order to provide flexibility and preserve farmland and resource protection areas. These provisions were amended once in 2002 and again a few years later, creating the program now controlled by Chapter 26 of the St. Mary’s County Comprehensive Zoning Ordinance in effect in November 2011.


The St. Mary’s County TDR program aims to preserve farmland and resource protection areas. Sending areas are parcels in Rural Preservation Districts (RPDs), a zone that allows only one dwelling unit per parcel, regardless of size, unless the site becomes a TDR receiving site as explained below. One TDR is allocated for every five acres of RPD-zoned gross land area. Sending site owners who choose to participate can choose to build on site or remove some or all of their development rights and transfer them to any person or legal entity.

TDRs can be used to increase residential density on parcels zoned RPD, RL, RH, RMX, VMX, TMX, CMX and RNC (in growth areas only.) The baseline density in these districts is one unit per acre with the exception of 10 units per acre in the RH and the special requirements that apply in the RPD (explained below.) In all but the RPD, one TDR increases residential density by one dwelling unit. The increased density allowed through TDR ranges from 1 bonus unit per acre in the RNC district to 14 bonus units per acre in the CMX district. On receiving sites zoned RPD, the first dwelling unit built on a receiving site does not require a TDR but uses five acres of base density. Thereafter less acreage is needed for each bonus lot but each bonus lot requires more TDRs as depicted in the following table.

Density (units per acre) Number of TDRs
1 dwelling unit per 5 acres 1 TDR per lot after the first lot or dwelling
1 dwelling unit per 4 acres 2 TDRs per lot or dwelling
1 dwelling per 3 acres 3 TDRs per lot or dwelling

TDRs can be also be used to increase non-residential intensity of land zoned RPD, RSC, RCL, RL, RMX, VMX, TMX, DMX, CMX CC, OBP and I at the ratio of 2,000 square feet of floor area in excess of a baseline FAR for each TDR.

Categories of receiving areas are paired with categories of sending areas. For example, receiving areas can only be located in a Critical Area if they use TDRs from sending areas in the Critical Area. Similarly, TDRs cannot be transferred to a Resource Conservation Area from within an Intensely Developed Area (IDA) or Limited Development Area (LDA) or from an IDA to an LDA.

St. Mary’s County allows developers to comply with TDR requirements using cash in lieu of TDRs. The amount is set at 120 percent of the average fair market value paid for TDRs in private market transactions in the previous year but the Board of County Commissioners can increase or decrease that amount. In 2011, the Board set the cash-in-lieu amount at $20,000. The County can use the cash-in-lieu payments to buy development rights or replenish the Critical Farms Program.

Program Status

In a 2007 study for Resources for the Future, Margaret Walls and Virginia McConnell reported that the original program that ended in 2002 transferred only 9 TDRs. The County Comprehensive Plan, adopted in 2010, reported that TDR had preserved 3,330 acres. In other words, the recent amendments have turned this into one of the most active TDR programs in the country. Walls and McConnell credit much of the program’s improved performance to the fact that St. Mary’s County has eliminated some of the non-TDR ways of obtaining bonus density. In addition, the changes made in the RPD zone could make a huge difference in program effectiveness. First the RPD only allows one dwelling per parcel without a TDR requirement. That new zoning suggests remarkable political determination to make this program work. After one TDR allows a second lot in the RPD, the third lot requires two TDRs and all subsequent lots require three TDRs each. This is a productive formula and it is operating at a density (one unit per three acres and less) with proven demand. Like the TDR program in Calvert County, the St. Mary’s County TDR program may succeed in saving a meaningful amount of farmland. By design, some farmland can be sacrificed to achieve this success. However the County may take the point of view that some loss of farmland is inevitable and that sacrificing some farmland to permanently preserve other farmland is a reasonable tradeoff.