I need help with CFraming a Part's Y Position?

You should thank me a lot since I basically wrote the script for you:

uis = game:GetService("UserInputService") --Getting input service
player = game.Players.LocalPlayer --Getting player
part = nil --Variable for your part
buildingHeight = 20 --Your building height (building's top Y coordinate)
building = game.Workspace.building --Your building


uis.InputBegan:connect(function(input) --Getting player's inputs
	if input.KeyCode == Enum.KeyCode.E then --Checking if E key was pressed
		part = Instance.new("Part") --Creating new part
		part.Anchored = true
		part.CanCollide = false --Optional property if you don't want player to be pushed by part
		part.Parent = game.Workspace --Getting part to workspace
		part.CFrame = player.Character.Torso.CFrame * CFrame.new(0,3,0) --Teleporting part 3 studs above player's torso
		part.Orientation = Vector3.new(0,0,0) --Resetting part's rotation
		if part.Position.Y > buildingHeight then --Checking if part is not above building height
			local difference = buildingHeight - part.Position.Y --Getting difference from heights
			part.CFrame = part.CFrame * CFrame.new(0,difference,0) --Subtracting difference from part's overall height
		end
		part.Position = Vector3.new(building.Position.X,part.Position.Y,building.Position.Z) --Teleporting part to building
	end
end)

This will get part in the building's center (add offset to the Vector3 in line 20 if you don't want the part to be in the building brick) 3 studs above player's torso's position or at the top of building if player is above it. It MUST be placed in a LocalScript to work, which has to be placed in the player's character's model (or StarterPack / StarterGui).