Irish juniper (Juniperus communis 'Stricta' or 'Hibernica') could work. Stays very narrow and doesn't get very tall. Other possibilities are Ilex crenata 'Sky Pencil' or Rhamnus frangula 'Fine Line'.

conifers are trees.. and trees basically never stop growing ...

most size estimates in the better conifer sources is at 10 years... and you can presume they will double at 20 years ...

relative size differences are because of the annual growth rate of any given plant... a plant that grows 1 foot a year.. will be 10 feet at 10 years ... if 6 inches.. then 5 feet ... and this growth rate is what gives you the best idea of how a conifer will fit in a given spot ...

https://conifersociety.org/conifers/learn/conifer-sizes-growth-rates/

you dont mention the size you saw ... but your base problem.. is that whatever grows to said height... in time to make you happy achieving your goal ... will easily surpass it..

sorry .. its not a specific answer ...

something like a Japanese maple might better fit the space... and be easily prunable ... to your wishes ...

ken

LOL!! One doesn't just prune a Japanese maple to "one's wishes". In fact the less pruning involved - other than some canopy thinning - the better.

Yeah, the growth thing, I find it very challenging to understand. Sometimes there is very contradictory information out there about a particular plant. Some trees and shrubs do seem to have a lifetime max, or significant slow down after some time. I have plenty of Japanese Maples, and plan to get more, but for that spot, need something dense and evergreen.

GardenGal, thank you for those suggestions. The Irish Juniper was not on my radar, but what would be the difference between that and the Blue Point? I'm seeing they both have the same 10 year expected growth. Also, would you know who carries them near Seattle?

That would probably be a little too slow growing...I'd have to find ones 10+ years old already in order for them to fulfill their purpose any time soon. I did find some videos last night of people pruning their Blue Point junipers. I'm debating between taking a chance with those, or the Ilex Sky Pencil (don't love the look as much but easy). Or possibly the Viburnum Laurustinus, if i can find any that aren't Spring Bouquet, though that would probably take too long to fill in also.

If you go with the viburnum, you will be needing to prune hard and often. It is not a "small" hedge plant :-)

Ok, good to know. Maybe I'll use that one for the other side where I won't have to be as careful about how tall it gets. This space, where we share a fence with the neighbors, will have to stay at a certain height all the time, ~6.5-8', so we don't shade their entire deck.

Upright Juniperus communis cultvars tend to eventually fall open in this region. Maybe this is a reason why they are not nearly as prevalent here as narrow growing kinds of other conifer species.

Most in Seattle are tight columns of prickly gray-blue foliage, usually called Irish or Swedish junipers. They are seen as yard trees or foundation-plantings and are sometimes tied with wire or string to keep their shape...

- A. L. Jacobson, Trees of Seattle - Second Edition (2006)

who goes on to list 4 addresses where examples could be seen at time of publication, all of them taller than 12' and starting with one 21' tall. Even a 'Compressa' at South Seattle Community College he mentions was 6' tall.

Based on material presented at local outlets 'Gold Cone' is clearly not a miniature growing only 3 in. per year either.

I you have a number of Japanese maples maybe continue the theme with something else Japanese. At any rate if you want to start off with some pretty good size already present that will be a major determinant for what you end up with. Cruise local garden centers to see what they might have for you, then pick something that seems right. This is more efficient then drawing up a list beforehand, perhaps only to discover that what you have chosen is not on the local market at the time you are shopping.