The architecture of Ottawa is most marked by the city's role as the national capital of Canada. This gives the city a number of monumental structures designed to represent the federal government and the nation. As a historic city, the most significant architecture in the city centre tends to be Gothic Revival, Romantic and other Picturesque styles of architecture, with the Parliament Buildings, dating from the 1850s, being an example of Gothic Revival. Other buildings in the city follow formalistic and functional architectural styles.

Ottawa has always had a mix of different architectural styles, varying considerably based on what era a building or neighbourhood was constructed in. While founded in the early nineteenth century, few buildings outside of the Byward Market, Ottawa's old town, survive from that era and the vast majority of the city's structures date from the twentieth century. Much of the downtown was also greatly transformed in the 1960s and 1970s, and the swath of suburbs that surround the city also date from this period. Most of the city's remaining 19th century buildings are in the Byward Market, Parliament Hill, Sandy Hill, New Edinburgh, and to a lesser extent in Centretown. Areas such as the Byward Market and Parliament Hill allow the city to retain a certain European character despite the dominance of modern architecture.

Unlike several other national capitals, such as Paris and Washington, D.C., Ottawa was never built to a master plan. However, several commissions have played a role in determining the shape of the city. Colonel By envisioned building several grand boulevards but the difficulties of expropriation and demolition prevented this from happening. In the late 1880s, Prime Minister Sir Wilfrid Laurier developed a 50-year vision of the city's future development and created the Ottawa Improvement Commission (OIC). The early years of the OIC under the direction of Montreal-born architect Frederick Todd saw the removal of industry along the canal, the definition of Patterson Creek and the transfer of Rockcliffe Park to the federal government. In 1913, Sir Robert Borden appointed the Sir Henry Holt Commission which was the first to state the need for a national capital region and also the removal of railway lines from the downtown core.

Twenty years later, the Federal District Commission and Prime Minister Mackenzie King urged the federal government to acquire land, which eventually led to the creation of Confederation Square. In 1939, King invited Jacques Gréber to create a master plan for the city. This plan proposed new parkways along the Ottawa and Rideau Rivers, included the idea for the Greenbelt, and urged the need for a ceremonial route to Parliament but the plan was not approved until 1951 by the government of Louis St. Laurent. Gréber's Plan has mostly been implemented and still affects the city today.

In 1958, the National Capital Commission (NCC) replaced the Federal District Commission. As a federal agency, the NCC worked along with were then many individual municipal governments on the Ontario and Quebec sides in planning and designing the city. It acquired the lands for the Greenbelt and in the 1960s it removed the railway tracks from downtown, making rail travel less accessible, but also opening the Rideau Canal as a scenic tourist destination. The NCC continues to have a major role in planning and does have a long range plan for how to showcase the region as a national capital, but has limited legitimacy as an unelected bureaucracy.

The streets of central Ottawa follow a grid pattern, but it is frequently disrupted by the Rideau River and Rideau Canal, ensuring that few streets in most of the older neighbourhoods are very long. Outside of the core, the roads follow the modern standard of large avenues forming a grid, interspersed with a network of crescents and cul-de-sacs which create low-traffic, suburban neighbourhoods. The Queensway, a major highway, crosses almost all of the city from east to west, going just south of downtown. It was built in the 1960s, mostly over former railway tracks, and thus did not entail the same urban destruction as expressways in other cities. There are five road bridges, and one rail bridge, crossing over the Ottawa River, four of which are in the downtown area, ensuring that much of the interprovincial traffic, including many large trucks, pass through the centre of town.

Several planning decisions have played an important role in the development of Ottawa and its architecture. One long standing rule that had a great effect on the downtown core, was a prohibition on buildings being taller than the 92 metres (302 ft) tall Peace Tower. It was instituted to prevent the Parliament Buildings from being dwarfed by more modern structures. Therefore Ottawa's central business district does not have the towering buildings found in most other North American cities, instead having a considerable number of mid-sized towers.This rule however is relaxed outside the downtown area, resulting in tall structures across the city, such as the Claridge Icon in Little Italy.

Ottawa is home to a large Greenbelt circling the entire urban core. It was created as an attempt to limit sprawl and encourage density, with mixed success. The Greenbelt has remained largely intact, but Ottawa's newest suburbs such as Kanata, Barrhaven, and Orleans have jumped over the belt. The Greenbelt is increasingly becoming a wide avenue of green between two developed areas. Prior to amalgamation in 2000 the region was divided into several communities each with its own planning guidelines and the suburbs have distinct characters. Kanata is especially notable as developer Bill Teron's attempt to create a modern suburb embracing garden city principles.