function isPointInPart(point, part)
local p = part.CFrame:PointToObjectSpace(point)
local s = part.Size/2
return p.X >= -s.X and p.X <= s.x and p.Y >= -s.Y ... etc
end

function checkValidity(pos)
local part = game.Workspace.BuildingStuff.BuildingArea
local p = part.CFrame:PointToObjectSpace(pos)
local s = part.Size/2
print(p.X)
print(-s.X)
print(s.X)
return (p.X >= -s.X and p.X <= s.X) and (p.Y >= -s.Y and p.Y <= s.Y) and (p.Z >= -s.Z and p.Z <= s.Z)
end

Don’t know what to tell you, it works fine for me. Can you try inserting this model (in a blank place, don’t insert untrusted models in your project places) and see if you can get it to work?

function isPointInPart(point, part)
local p = part.CFrame:PointToObjectSpace(point)
local s = part.Size/2
return (p.X >= -s.X and p.X <= s.X) and (p.Y >= -s.Y and p.Y <= s.Y) and (p.Z >= -s.Z and p.Z <= s.Z)
end
while wait() do
local c = isPointInPart(script.Parent.Point.Position, script.Parent.Volume) and BrickColor.Green() or BrickColor.Red()
script.Parent.Point.BrickColor = c
end

This is my solution to the problem. It’s a little messy, I guess, but it, so far, has never failed me.

local a = workspace.Part1 -- The part in who's region you want the position to be in.
local b = Vector3.new(30,45,10) -- The position you want to check. (I just put a random one for an example)
-- The following equations will return you a Vector3Value which describes the distance of position B from the position of part A, while also considering the LookVector, RightVector, UpVector, etc.
local checkZAxis = a.CFrame.LookVector*(b - a.Position)
local checkXAxis = a.CFrame.RightVector*(b - a.Position)
local checkYAxis = a.CFrame.UpVector*(b- a.Position)
-- From here you will have to individually check to see if the part is within each axis.
if math.floor(math.abs(checkXAxis.X + checkXAxis.Z)) < a.Size.X/2 then -- You will have to add both the X and Z axes for the returned checkXAxis Vector3 to get the magnitude. Same goes for the checkZAxis Vector3.
if math.floor(math.abs(checkYAxis.Y)) < a.Size.Y/2 then
if math.floor(math.abs(checkZAxis.X + checkZAxis.Z)) < a.Size.Z/2 then
-- The point is within part A.
end
end
end

script.Parent.Touched:Connect(function(Hit)
local bool = false
local mag = (script.Parent.Position - Hit.Position).Magnitude
if Hit.Position.X > mag or Hit.Position.Y > mag or Hit.Position.Z > mag then
bool = true
print(bool)
end
end)

The issue is that it DOES work, outside this script. But it doesn’t make sense since nothing in the script is interfering with it. Perhaps the inputs are incorrect? I’ll try some stuff.
Also @Ai_Kyle it appears it has failed me rather than you lol

All these solutions offered should work fine. I even tested mine out several times to make sure it worked properly. If none of them are working, then maybe your issue has to do with something else.