Script

-- MODULE SCRIPT
local IsServer = game:GetService("RunService"):IsServer()

local Placement = {}
Placement._index = Placement

function Placement.New(surface)
    local self = setmetatable({}, Placement)
    -- Describes what part is the surface
    self.Surface = surface

    -- Objects
    if (IsServer) then
        self.Objects = surface.Objects
    else
        self.Objects = game:GetService("ReplicatedStorage"):WaitForChild("Remotes").StartPlacement:InvokeServer(surface)
    end
    -- Describes what face objects will be placed on
    self.PlacingSurface = Enum.NormalId.Top
    -- How big each side of 1 grid unit is. Must be a divisible of the placing surface X and Z
    self.UnitStep = 1
    return self
end

function Placement:CalcSurfaceConstraints()
    local SurfaceSize = self.Surface.Size

    -- Make a CFRame where LookVector = UpVector
    -- Done by using Object space to build a CFrame

    local Back = Vector3.new(0, -1, 0) -- Z Axis vector
    local Top = Vector3.new(0, 0, -1) -- Y Axis vector
    local Right = Vector3.new(-1, 0, 0) -- X Axis vector

    -- Convert Back, Top and Right to a World Space CFrame
    local CF = self.Surface.CFrame * CFrame.fromMatrix(-Back*SurfaceSize/2, Right, Top, Back)
    -- Uses Object space vectors to find the width and height of the placement surface in whole numbers
    local Size = Vector2.new((SurfaceSize * Right).magnitude, (SurfaceSize * Top).magnitude)

    return CF, Size
end

function Placement:CalcPlacement(Model, Pos, Rot)
    -- Uses other method to get info about the placing surface
    local CF, Size = Placement:CalcSurcafeConstraints()
    -- Rotates the model size to properly constrain to the surface
    local ModelSize = CFrame.fromEulerAnglesYXZ(0, Rot, 0) * Model.PrimaryPart.Size
    ModelSize = Vector3.new(math.abs(ModelSize.X), math.abs(ModelSize.Y), math.abs(ModelSize.Z))
    -- Get the model position relative to the surface CFrame
    local LPos = CF:PointToObjectSpace(Pos)
    -- Max distance model can be from surface center
    local Size2 = (Size - Vector2.new(ModelSize.X, ModelSize.Z))/2
    -- Constraint the model position so you can't drag it off the base.
    local X = math.clamp(LPos.X, -Size2.X, Size2.X)
    local Y = math.clamp(LPos.Y, -Size2.Y, Size2.Y)
    -- Create the placement CFrame and return the paramters
    return CF * CFrame.new(X, Y, -ModelSize.Y/2) * CFrame.Angles(-math.pi/2, Rot, 0)
end

return Placement

-- LOCAL SCRIPT
local Placement = require(game.ReplicatedStorage.PlacementModule)
local placement = Placement.New(game.Workspace.LandPlots.LandPlot1.Base)

local CF, Size = placement:CalcSurfaceConstraints()
print(CF, Size)

-- ERROR FROM LOCAL SCRIPT
  21:15:38.298 - Players.ilostmyaccount678.PlayerGui.LocalScript:4: attempt to call a nil value