Boulder County, Colorado

Boulder County, Colorado, population 326,196 (2019), begins roughly 15 miles northwest of Denver’s downtown. The eastern third of the county extends into the Great Plains and the western two-thirds encompasses portions of the Rocky Mountains with extensive public lands including the Roosevelt National Forest, Rocky Mountain National Park, and several other parks, preserves, and wilderness areas.

Boulder County is an open space super star. As profiled in Lasting Value: Open Space Planning and Preservation Successes, in addition to 137,308 acres held by the federal government, Boulder County itself protects 105,386 acres by ownership, lease and easement. The county uses various preservation techniques including voter-approved open space sales taxes, property taxes, grants, and TDR mechanisms that feature a remarkable amount of voluntary intergovernmental cooperation.

In 1981, Boulder County began motivating open space preservation by adopting a clustering option known as non-urban planned unit development, or NUPUD. This mechanism allows base density to be doubled, from one unit per 35 acres to two units per 35 acres, when development is concentrated on no more than 25 percent of the parcel and the remaining 75 percent is preserved by a permanent conservation easement.

In 1989, Boulder County added a modification that created a transfer of development rights option known as non-contiguous non-urban planned unit development, or NCNUPUD. The county used this technique because TDR was not then explicitly authorized by Colorado state law. In this process, one PUD application allows the transfer of development potential between two non-contiguous parcels. 

According to Article 6 of the Boulder County Land Use Code as of June 2019, Planned Unit Developments, the sending and receiving parcels of a NCNUPUD must be located within the same water service area and/or water and/or sanitary district, school district, and fire district response area unless mitigation measures are adopted to address development imbalances. On sending sites, most of the land proposed for preservation must be identified in Boulder County’s Comprehensive Plan as having agricultural, natural, cultural, environmental, scenic, community buffer or open space significance. The receiving sites cannot be located within areas likely to be annexed by a municipality without the approval of that jurisdiction. 

NCNUPUD can triple the density permitted by NUPUD. For example, a 35-acre parcel is allowed one unit as a matter of right, two units when the NUPUD option is used, or six units via NCNUPUD. No more than half of the receiving site can be developed. If the receiving site contains agricultural lands of national importance, no more than 25 percent of the parcel may be developed. If at least half of the units are transferred in the original transfer, the remaining unit can be held for use in subsequent applications as long as various conditions are met.  

Boulder County added a second TDR tool called Transferred Development Rights Planned Unit Development (TDR/PUD) with the purpose of preserving generally contiguous properties to protect agriculture, rural character, open space, scenic vistas, natural features, and environmental resources. The county maps some sending areas as well as the sending and receiving areas for Niwot, an unincorporated urbanized area. In addition, sending areas can be designated by Inter Governmental Agreements between the county and municipalities. The code allows the transfer of various types of development potential: residential and yard area minimums. The code also permits conversions from residential to non-residential uses. At least 75 percent of the development transferred to a receiving site must come from a sending site within the same designated subarea unless the county board grants an exception. Receiving sites must be located adjacent to an arterial, collector, or transit route and the receiving site development must be compatible with adjacent development.

Sending sites must be protected by a conservation easement on the entire parcel and no more than five percent of the sending property may be developed. Subject to a public hearing, TDRs can be severed and sold from land acquired in fee by a governmental entity or protected by an easement held by a governmental entity. The of development allowed to remain on sending sites versus transferred to a receiving site varies based on the size of the sending property. For parcels between 35 and 52.49 acres, two development rights can be transferred or one unit can be transferred and the other unit built on site as long as onsite construction is consistent with the comprehensive plan. In the last of seven categories, parcels 140 acres and larger can transfer two TDR per 35 acres or any combination which does not exceed a transfer of two units per 35 acres or one unit per 70 acres on site.

Boulder County allows property owners with deliverable agricultural water rights to transfer one additional unit per 35 acres if the owner provides an undivided interest in those water rights to the county. 

Conservation easements on sending sites state that the easement can be terminated if termination is consistent with the comprehensive and management plans of Boulder County and other interested governmental entities. However, the county may require compensation for easement termination in an amount sufficient to offset any loss of public benefit.    

As explained above, the TDR/PUD process can work interjurisdictionally as well as intra-jurisdictionally. Boulder County uses inter-governmental agreements (IGA) to facilitate the transfer of development potential from sending sites within unincorporated Boulder County to receiving sites within incorporated cities. This process was tested in the Boulder Valley TDR Program, implemented through an inter-governmental agreement between the City of Boulder and Boulder County adopted in 1995. At last count, Boulder County had signed IGAs with eight municipalities and the unincorporated town of Niwot.  

In a classic interjurisdictional program, a planning area is created around the community participating in the TDR program with the county. In this way, the community can maximize the benefits of open space preservation by requiring that a minimum percent of transferred development rights come from the rural areas immediately surrounding them. The planning area may be further divided into subareas with the transfers required to occur between sending and receiving sites within that subarea. With this technique, the neighborhoods accepting the extra receiving site development also are closest to the open space which that extra density made possible

While certain procedures remain constant, individual requirements are spelled out by the separate IGAs. The most detailed IGA, adopted in April 1995 as the “Boulder Valley TDR Comprehensive Development Plan”, is between Boulder County and the City of Boulder.

In general, this IGA combines the City of Boulder’s commitment to accept transferred development rights from the County and the County’s commitment to preserve rural character. More specifically, the County agrees not to approve NCNUPUDs within the Plan Area or NUPUDs within the Planning Reserve Area unless they are jointly approved by both the City and County. Correspondingly, the City agrees not to annex unincorporated land or otherwise allow development that is contrary to the jointly-adopted Boulder Valley Comprehensive Plan (BVCP).

The TDR Plan identifies four categories of sending sites: 1) the Rural Preservation Area; 2) the Accelerated Open Space Acquisition Area; 3) the Northern Tier Lands; and 4) Private Land Enclaves lying between the Boulder Mountain Parks and the Arapaho-Roosevelt National Forest west of the City. Receiving sites can include land within the boundaries of the City of Boulder’s community service area, areas being annexed to the City in accordance with the provisions of the BVCP or lands within a rural planning area that have been approved under the provisions of the BVCP. Another area, Planning Reserve Area, is suitable for urban development in the long-term future; but in the near term it is planned to remain rural and cannot be used as a receiving site.

Under the IGA, the City of Boulder agrees to accept up to 250 development rights. Certificates of Development Rights are issued only after a conservation easement, precluding further development and granted jointly to the County and City, is recorded for the sending site. The IGA further requires the City and County to establish a joint committee to monitor the progress of the TDR program. The Agreement also prevents the County from making any changes to its NUPUD or NCNUPUD regulations without the City’s consent. Finally, the IGA between Boulder City and Boulder County terminates five years after its effective date.

The IGA between the County and the City of Lafayette, which became effective in December of 1995, is similar to the IGA with the City of Boulder. However this IGA does not limit the number of development rights which can be transferred to Lafayette. In addition, this TDR plan clearly limits the sending sites to designated areas within a 27-square-mile Plan Area that extends from one to four miles in each direction from the Lafayette City Limits. The IGA with the City of Longmont, which became effective in January of 1996, similarly limits the sending area to about 50 square miles at the periphery of the Longmont TDR Planning Area. 

As of 2009, TDR in Boulder County had preserved between 4,400 and 5,900 acres (Pruetz & Standridge 2009). However, TDR activity had declined by 2005 because there were fewer and fewer parcels of 35 acres or greater that qualified as sending sites. At that time, over 85 percent of county land was either within incorporated communities or already preserved by public ownership or conservation easements resulting from federal and state programs as well as the many preservation options launched by Boulder County and its municipalities (Fogg 2005). 

In 2008, Boulder County adopted a new program which requires developers to acquire transferable development credits (TDCs) to exceed a floor area baseline of 6,000 square feet within individual dwelling units. This 6,000-square foot baseline includes garages, basements and residential accessory structures. Each TDC allows an additional 500 square feet of floor area until reaching 1,500 bonus square feet of floor area; above that 1,500 square foot increment, two TDCs are needed for every 500 square feet of additional floor area. (Under limited circumstances, TDRs from the TDR/PUD program discussed above can be converted to TDCs.)

Sending sites for the TDC program are legal building lots with legal access. Property owners who choose to participate receive the following TDCs depending on the permanent restrictions they record on their properties. 

  • 2000 square feet of floor area or less: 2 TDCs
  • 1500 square feet or less: 3 TDCs
  • 1000 square feet or less: 4 TDCs
  • Keep land vacant: 5 TDCs for mountains properties and 10 TDCs for plains properties.

This program aims to preserve rural character, motivate affordable housing, and promote more sustainable development by offsetting the impacts of larger houses with the preservation of smaller houses and the open space resulting from smaller homes. As of January 2021, 251 TDCs had been sold at prices ranging from $3,000 to $10,000 per TDC, with most sales occurring toward the high end of that range.


Fogg, P. 2005. Correspondence with author: February 9 2005.

Pruetz, R. and N. Standridge. 2009. What Makes Transfer of Development Rights Work? Success Factors From Research and Practice. Journal of the American Planning Association. Vol. 75, No. 1, Winter 2009.