BuildScript.cs

using UnityEngine;
using System.Collections;

public class BuildScript : MonoBehaviour {

    GameObject GroundObject;
    bool onGround;
    Vector3 speed;
    float originalMass;
    System.Single zero = 1e-7f;
    Vector3 rotation;
    Vector3 lastHit;

    // Use this for initialization
    void Start () {
        GroundObject = null;
        onGround = false;
        lastHit = new Vector3(0, 0, -100);
    }

    void OnCollisionEnter(Collision collision){

    }

    // Update is called once per frame
    void FixedUpdate () {
        speed  = new Vector3(3, 0, 0);
        int x = Screen.width / 2;
        int y = Screen.height / 2;
        Ray ray = Camera.main.ScreenPointToRay(new Vector3(x, y));
        RaycastHit hit;
        Transform cam = Camera.main.transform;
        float snap = 15;

        // Release object when pressing left mouse button
        if(GroundObject != null && Input.GetMouseButtonDown(0)){
            GroundObject.layer = 0;
            GroundObject.rigidbody.isKinematic = false;
            GroundObject.rigidbody.mass = originalMass;
            GroundObject.rigidbody.freezeRotation = false;
            GroundObject = null;
        }

        // Get GameObject when pressing right mouse button
        if(GroundObject == null){
            if(Physics.Raycast(ray, out hit) && hit.distance < 5 && hit.transform.gameObject.rigidbody != null && Input.GetMouseButtonDown(1)){
                GroundObject = hit.transform.gameObject;
                originalMass = GroundObject.rigidbody.mass;
                GroundObject.rigidbody.mass = zero;
                GroundObject.layer = 2;
                GroundObject.rigidbody.rotation = Quaternion.identity;
                GroundObject.rigidbody.freezeRotation = true;
                GroundObject.rigidbody.isKinematic = true;
            }
        }

        if(GroundObject != null){
            if(GroundObject.rigidbody.isKinematic == false){
            GroundObject.rigidbody.velocity = Vector3.zero;
            }

            // Calculate GroundObjects position when on ground
                if (Physics.Raycast(ray, out hit) && hit.distance < 5) {
                    GridController grid = (GridController) GameObject.Find("GridController").GetComponent(typeof(GridController));
                    if (lastHit != hit.point){
                        grid.Generate(hit.point);
                    }
                    lastHit = hit.point;
                    GroundObject.rigidbody.MovePosition(grid.snapPoint(GroundObject) + speed * Time.deltaTime);
                //  grid.Clear();
                    onGround = true;
                }else{
                onGround = false;
            }

            // Rotate object with mouse when right mouse button is down
            if(Input.GetMouseButton(1)){
                if (Physics.Raycast (Camera.main.ScreenPointToRay(Input.mousePosition), out hit))
                {
                    Vector3 roundAngles = new Vector3(Input.GetAxis("Mouse Y") * 2, Input.GetAxis("Mouse X") * 2, 0);
                    roundAngles.x = Mathf.Round(roundAngles.x / snap) * snap;
                    roundAngles.y = Mathf.Round(roundAngles.y / snap) * snap;
                    roundAngles.z = Mathf.Round(roundAngles.z / snap) * snap;
                    GroundObject.transform.Rotate(roundAngles, Space.World);

                }
            }
            if(GroundObject.transform.rotation != Quaternion.identity){
                Vector3 rotation = GroundObject.transform.eulerAngles;
                rotation.x = Mathf.Round(rotation.x / snap) * snap;
                rotation.y = Mathf.Round(rotation.y / snap) * snap;
                rotation.z = Mathf.Round(rotation.z / snap) * snap;
                GroundObject.transform.eulerAngles = rotation;
            }
            }
    }

    void LateUpdate () {
        if(GroundObject != null){
        // Set object in coordinates set in editor
        if(onGround == false){
            foreach (Transform child in transform)
            {
                if(child.name == "BuildObject_1"){
                    GroundObject.rigidbody.isKinematic = true;
                    GroundObject.transform.position = child.transform.position;
                }
            }
        }
        // Snapping coordinates
        if (onGround == true){

        }
    }
    }
}