How do I insultingly describe an extremely ugly building?

An eyesore:

• A thing that is very ugly, especially a building that disfigures a landscape.

(ODO)

also:

a blot on the landscape:

• something such as an ​ugly ​building that ​spoils a ​pleasant ​view.

The expression architectural aberrations is also used:

(The New York Times)

Ngram: architectural aberrations

eyesore

A thing that is very ugly, especially a building

http://www.oxforddictionaries.com/definition/english/eyesore

There is the term Brutalist, which strictly covered only such buildings built in the period between the 1950s and the mid-1970s, but which has since become genericised (and lost the capital letter). Wikipedia is, as usual nowadays, spot on:

Brutalist architecture is a movement in architecture that flourished from the 1950s to the mid-1970s, descending from the modernist architectural movement of the early 20th century. The term originates from the French word for "raw" in the term used by Le Corbusier to describe his choice of material béton brut (raw concrete). British architectural critic Reyner Banham adapted the term into "brutalism" (originally "New Brutalism") to identify the emerging style.

Brutalism became popular with governmental and institutional clients, with numerous examples in Britain, France, Germany, Japan, the United States, Canada, Brazil, the Philippines, Israel and Australia. Examples are typically massive in character (even when not large), fortress-like, with a predominance of exposed concrete construction, or in the case of the "brick brutalists," ruggedly combine detailed brickwork and concrete. There is often an emphasis on graphically expressing in the external elevations and in the whole-site architectural plan the main functions and people-flows of the buildings. Brutalism became popular for educational buildings (especially university buildings), but was relatively rare for corporate projects. Brutalism became favoured for many government projects, high-rise housing, and shopping centres to create an architectural image that communicated strength, functionality, and frank expression of materiality.

In its ruggedness and lack of concern to look comfortable or easy, Brutalism can be seen as a reaction by a younger generation to the lightness, optimism, and frivolity of some 1930s and 1940s architecture. In one critical appraisal by Banham, Brutalism was posited not as a style but as the expression of an atmosphere among architects of moral seriousness. "Brutalism" as an architectural critical term was not always consistently used by critics; architects themselves usually avoided using it altogether. More recently, "brutalism" has become used in popular discourse to refer to buildings of the late twentieth century that are large or unpopular – as a synonym for "brutal."

abomination

The noun abomination means a thing or action that is vile, vicious or terrible. For example, if you see a neighbor kick an old blind dog that's done nothing wrong, you might remark, "That kind of cruelty is an abomination!"

The word abomination comes from the Old French word abominacion meaning "horror, repugnance, disgust." The word abomination can also be used to refer to a person or object you find utterly loathsome and repellent. If you look at someone else as an abomination, that means you are horrified by them, and barely even think they're human. If you hear a friend describing you as an abomination behind your back, you need to find a new buddy right away! Vocabulary.com

atrocity

: the quality or state of being atrocious M-W

atrocious: very bad; abominable or disgusting : their taste in clothes is just atrocious Wiktionary