Moving an object on Y-axis in-game in grid based building game.

Hello everyone! I was hoping someone could help with the following:

I am following an In-game Building System tutorial on YouTube. With this system I can instantiate objects in-game and move and place them on a grid, along the Z- and X-axes.

Now I want to be able be able to move the object on the Y-axis (while still snapping to the grid) while holding down the Shift-button.

I've looked around online but I can't find it anywhere. The tutorial also doesn't cover it.

Does anyone know how to do this?

 using System.Collections;
 using System.Collections.Generic;
 using UnityEngine;
 using UnityEngine.UI;
 
 public class BuildingManager : MonoBehaviour
 {
     public GameObject[] objects;
     public GameObject pendingObject;
     [SerializeField] private Material[] materials;
 
     private Vector3 pos;
     private RaycastHit hit;
     [SerializeField] private LayerMask layerMask;
 
     public float rotateAmount;
     public float gridSize;
 
     bool gridOn = true;
 
     public bool canPlace = true;
     [SerializeField] private Toggle gridToggle;
 
     // Start is called before the first frame update
     void Start()
     {
 
     }
 
     // Update is called once per frame
     void Update()
     {
         if (pendingObject != null)
         {
             UpdateMaterials();
             if (gridOn)
             {
                 pendingObject.transform.position = new Vector3(
                     RoundToNearestGrid(pos.x),
                     RoundToNearestGrid(pos.y),
                     RoundToNearestGrid(pos.z)
                     );
             }
             else { pendingObject.transform.position = pos; }
 
             if (Input.GetMouseButtonDown(0) && canPlace)
             {
                 PlaceObject();
             }
             if (Input.GetKeyDown(KeyCode.Z))
             {
                 RotateObject();
             }
         }
     }
 
         public void PlaceObject()
     {
         pendingObject.GetComponent<MeshRenderer>().material = materials[2];
         pendingObject = null;
     }
 
     public void RotateObject()
     {
         pendingObject.transform.Rotate(Vector3.up, rotateAmount);
     }
 
     private void FixedUpdate()
     {
         Ray ray = Camera.main.ScreenPointToRay(Input.mousePosition);
 
         if (Physics.Raycast(ray, out hit, 1000, layerMask))
         {
             pos = hit.point;
         }
     }
 
     void UpdateMaterials()
     {
         if (canPlace)
         {
             pendingObject.GetComponent<MeshRenderer>().material = materials[0];
         }
         if (!canPlace)
         {
             pendingObject.GetComponent<MeshRenderer>().material = materials[1];
         }
 
     }
     public void SelectObject(int index)
     {
         pendingObject = Instantiate(objects[index], pos, transform.rotation);
     }
 
     public void ToggleGrid()
     {
         if (gridToggle.isOn)
         {
             gridOn = true;
         }
         else { gridOn = false; }
     }
 
     float RoundToNearestGrid(float pos)
     {
         float xDiff = pos % gridSize;
         pos -= xDiff;
         if (xDiff > (gridSize / 2))
         {
             pos += gridSize;
         }
         return pos;
     }
 }

How would you do this?