Placement System

local player = game:GetService("Players").LocalPlayer
local mouse = player:GetMouse()
local TweenService = game:GetService("TweenService")

local block = Instance.new("Part", workspace)
block.Anchored = true
block.Size = Vector3.new(4, 4, 4)
block.TopSurface = "Smooth"
block.BottomSurface = "Smooth"
block.BrickColor = BrickColor.new("Bright green")
block.Material = "Grass"
block.CanCollide = false
block.Transparency = 0.5

function round(pos, offset)
    local result = {}
    for i, val in pairs{X=pos.X;Y=pos.Y;Z=pos.Z} do
        local def = val - (val%offset)
        result[i] = def
    end
    local finalPos = Vector3.new(result.X, result.Y, result.Z)
    return finalPos + (Vector3.new(offset,offset,offset)/2)
end

local lastPos = Vector3.new()
local tween

local offset = 4
local REACH = 100

while wait() do
    local unitRay = mouse.UnitRay

    local castParams = RaycastParams.new()
    castParams.FilterDescendantsInstances = { player.Character , block}

    -- automatically ignores the player's character

    local result = Workspace:Raycast(
        unitRay.Origin, unitRay.Direction * REACH,
        castParams
    )

    if result then
        local newPos = round(result.Position + result.Normal * 1.5, offset)
        if (lastPos-newPos).magnitude > .5 and tween then
            tween:Cancel()
        end
        lastPos = newPos
        tween = TweenService:Create(block, TweenInfo.new(0.1), {Position = newPos})
        tween:Play()
    end
end