Take the image of this building for instance, a good percentage of people would say that it's not pleasant to look at. No colorful tiles, no fancy arcs. In short, it's "harsh-looking." I'm looking for a word that best describes buildings such as this one which are not aesthetically pleasing. Buildings that give off an ominous vibe or make you feel uneasy. If you search photos of headquarters of most intelligence agencies, naturally you'll see that their exteriors are very unwelcoming and harsh. As you can tell from the explanation I've provided, I've used the word "harsh" twice! Not that I think it doesn't get the job done, it does, but I'm looking for a more precise word.
This building has a specific architecture related to it. The word is brutalist.
Brutalist architecture was a design style that first used concrete in large buildings, where the concrete forms created the style and pattern of the building's external features. It favored "strong" large boxy shapes, was popular in the 1950's, and is now considered by some to be very ugly.
The name is related to the word "brutal" which comes from the same Latin root word "brutus". Brutal means
savage or violent
The second definition
punishingly hard or uncomfortable
The second definition of brutal closely describes some of the elements you are trying to express. "Its brutal square windows gave no sign of the life inside."
An appropriate word would be austere
stern and cold in appearance or manner
markedly simple or unadorned
An austere building is one that is simple, without any fancy adornments, and gives the sense of being harsh, or unwelcoming.
I'd also note that aesthetic is subjective; some people find this architectural style to be very pleasing.
I'm not sure if it's a colloquialism local to Ireland, but this is what we would call "Institutional".
That is having a no-frills functional design, built cheaply disregarding any care for how it looks, generally housing a state institution for something distasteful such as Sewage Treatment Plants, Prisons for dangerous criminals, Revenue Collectors . . .
utilitarian, unadorned, plain, grey, gloomy, glum, washed-out, tragic, bunker, fortress, slapped-together, uninviting, imposing, unwashed, weathered, armored, abandoned, run-down, neglected, unassuming.
It might be better to use more than one word. I like "an unadorned, weathered, industrial building"
Bringing in aesthetic values doesn't paint a good picture because tastes change and every person has their own idea of what beautiful is. Visit the modern art section at the Met.