ConstructionController.cs

using System.Collections;
using System.Collections.Generic;
using UnityEngine;

public class ConstructionController : MonoBehaviour
{
    private bool isBuilding = false;

    private GameObject item;

    [SerializeField]
    private GameObject itemPrefab;

    private LayerMask terrainLayermask;

    [SerializeField]
    private float maxDistance = 12f;

    [SerializeField]
    private GameObject placementIndicatorPrefab;

    private GameObject placementIndicator;

    private float smooth = 10f;

    private void Awake()
    {
        terrainLayermask = 1 << 17;
        isBuilding = false;
    }

    // Update is called once per frame
    void Update()
    {
        // Enable building mode
        if (Input.GetKeyDown(KeyCode.E))
        {
            isBuilding = !isBuilding;
            if (isBuilding)
            {
                placementIndicator = Instantiate(placementIndicatorPrefab, transform.position + Vector3.forward * maxDistance, transform.rotation);
            }
        }

        // While building mode is enabled...
        if (isBuilding)
        {

            // Place item
            if (Input.GetKeyDown(KeyCode.Alpha1))
            {
                item = Instantiate(itemPrefab, Camera.main.transform.position + transform.forward * maxDistance, transform.rotation);
            }

            if (item != null) {
                Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);
                RaycastHit hit;
                if (Physics.Raycast(ray, out hit, maxDistance, terrainLayermask))
                {
                    Vector3 offset = item.transform.position - hit.point;
                    item.transform.position = Vector3.Lerp(item.transform.position, hit.point, Time.deltaTime * smooth);
                }
            }
        }

        // Cancel building mode
        if (Input.GetKeyDown(KeyCode.Escape))
        {
            if (isBuilding)
            {
                isBuilding = false;
                Destroy(item);
                Destroy(placementIndicator);
            }
        }
    }

    private void OnDrawGizmos()
    {
        Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);
        RaycastHit hit;
        if (Physics.Raycast(ray, out hit, Mathf.Infinity, terrainLayermask))
        {
            Gizmos.DrawWireSphere(hit.point, 1f);
        }
    }
}