That is almost nothing! 2897 btus wouldn’t even do a small bedroom. 1707 btus according to what calculations? The pipes should surely be straight up to valves. Rads can be chosen to suit joints positions. I really doubt it is difficult Get a heating installer rather than a builder.
1707 BTU is about 0.5 KW, so 1/3rd the size of a typical radiator for an average room. Always best to go large with Rads, quicker heating and lower temperatures.
What size is the room - length, width and height? How many outside walls? What is insulation like in outside walls and floors and ceilings (if applicable)? New built well insulated homes have perhaps a third of the heat requirement in most rooms compared with a house of 50 plus years ago, but you still need to allow for condensing boilers working on lower temperatures and also severe winter weather. Generally only a few pounds extra to go a few sizes up in a radiator choice.
Also, apart from BTU rating of rad, that tiny size in that room and under that window looks, frankly ridiculous Ok, that’s looking from a cosmetic angle so perhaps, not the most important feature but....., it just looks plain stupid Even if a longer rad is over specced in terms of output for that room, you could turn it down with a TRV and the whole thing would look a lot better anyway (also sorting out the pipes neatly of course) At the moment, rad looks like it’s come outta a dolls house and been put into a real house
Something like a Stelrad Softline 600 x 1600 double panel plus P+ would look so much better and give out more than ample heat.
Room is 2.6x2.9x1.9. Two outside walls, 102mm brick, 100mm block, 100mm Rockwool. 0.28w/m2K. 100mm insulation above in pitched roof, 100mm insulation to room below. Fitting a bigger rad would get around the noggin issue as well.