A simple modular building generator for Unity.

SimpleModularBuildingGenerator.cs

/**
 * @brief   A simple modular building generator for Unity.
 * @author  Benjamin Williams 
*/

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

public class SimpleModularBuildingGenerator : MonoBehaviour
{
    /// <summary>
    /// A list of building spawnpoints
    /// </summary>
    public List<Transform> buildingSpawnpoints;

    [Header("[prefabs]")]

    /// <summary>
    /// The list of bottom prefabs
    /// </summary>
    public List<GameObject> bottomPrefabs;

    /// <summary>
    /// The list of top prefabs
    /// </summary>
    public List<GameObject> topPrefabs;

    /// <summary>
    /// The list of mid prefabs
    /// </summary>
    public List<GameObject> midPrefabs;

    [Header("[generation options]")]

    [Range(1,5)]
    public int maxMidTiers = 1;

    /// <summary>
    /// The roof rotation fix (optional)
    /// </summary>
    public Vector3 roofRotationFix;

    /// <summary>
    /// Just gives us a random selection from a list
    /// </summary>
    /// <param name="collection">The collection</param>
    /// <returns>A random item</returns>
    private T RandomChoice<T>(in List<T> collection) => collection[Random.Range(0, collection.Count)];

    /// <summary>
    /// Spawns the top-most tier
    /// </summary>
    /// <returns></returns>
    private GameObject SpawnTopTier(Transform spawn, GameObject lastMidTier)
    {
        //Choose a random prefab
        var randomTop = this.RandomChoice(in topPrefabs);

        //Find spawn position
        var spawnPosition = lastMidTier.transform.position + Vector3.up * lastMidTier.GetComponent<Renderer>().bounds.size.y;

        //Spawn it
        var obj = GameObject.Instantiate(randomTop, spawnPosition, spawn.rotation, spawn) as GameObject;

        //Set the name
        obj.name = "bottom-section";

        //OPTIONAL: Roof rotation fix
        obj.transform.Rotate(roofRotationFix);

        //Delete mesh renderer of the parent object so we dont see it (kinda optional)
        Destroy(spawn.gameObject.GetComponent<Renderer>());

        //Return the base
        return obj;
    }

    /// <summary>
    /// Spawns a random number of mid tiers on top of a base
    /// </summary>
    private GameObject SpawnMidTiers(Transform spawn, int numMidTiers, GameObject baseObject)
    {
        //Run from 0 to mid tiers
        //..

        //Set yOffset to base height, initially (we want to spawn on top of the base)
        float yOffset = baseObject.GetComponent<Renderer>().bounds.size.y;

        //The last mid tier spawned (for returning)
        GameObject lastMidTier = baseObject;

        for(int i = 0; i < numMidTiers; i++)
        {
            //Compute the place to spawn this mid tier:
            //It's basically just the base position + up * height of building currently
            var spawnPos = spawn.transform.position + Vector3.up * yOffset;

            //Choose a prefab
            var randomPrefab = this.RandomChoice(midPrefabs);

            //Spawn at this location, set name
            var obj = GameObject.Instantiate(randomPrefab, spawnPos, baseObject.transform.rotation, spawn);
            obj.name = $"mid-tier-{i}";

            //Add to yOffset the height of this spawned mid tier
            yOffset += obj.GetComponent<Renderer>().bounds.size.y;

            //Sete last mid tier to this
            lastMidTier = obj;
        }

        //Return the last mid tier
        return lastMidTier;
    }

    /// <summary>
    /// Spawns a base at a particular location
    /// </summary>
    /// <param name="spawnTransform"></param>
    private GameObject SpawnBase(Transform spawnTransform)
    {
        //Choose a random prefab
        var randomBottom = this.RandomChoice(in bottomPrefabs);

        //Spawn it
        var obj = GameObject.Instantiate(randomBottom, spawnTransform.position, spawnTransform.rotation, spawnTransform) as GameObject;

        //Set the name
        obj.name = "bottom-section";

        //Delete mesh renderer of the parent object so we dont see it (kinda optional)
        Destroy(spawnTransform.gameObject.GetComponent<Renderer>());

        //Return the base
        return obj;
    }

    // Start is called before the first frame update
    void Start()
    {
        foreach(var spawn in buildingSpawnpoints)
        {
            //Spawn the base
            var baseObject = this.SpawnBase(spawn);

            //Choose a random amount of mid tiers
            int numMidTiers = Random.Range(0, maxMidTiers);

            //And spawn that many
            var lastMidTier = this.SpawnMidTiers(spawn, numMidTiers, baseObject);

            //Spawn a roof
            this.SpawnTopTier(spawn, lastMidTier);
        }
    }


    private float lastTime = 0;
    public float updateDelta = 0.25f;

    void Update()
    {
        //Clamp one-sided
        updateDelta = Mathf.Max(updateDelta, 0.05f);

        if((Time.time - lastTime) > updateDelta)
        {
            //Set last time
            lastTime = Time.time;

            foreach(var spawn in buildingSpawnpoints)
            {
                //Kill all children in building spawn points
                //..

                foreach(Transform child in spawn)
                    Destroy(child.gameObject);

                //Respawn!
                //..
                
                //Spawn the base
                var baseObject = this.SpawnBase(spawn);

                //Choose a random amount of mid tiers
                int numMidTiers = Random.Range(0, maxMidTiers);

                //And spawn that many
                var lastMidTier = this.SpawnMidTiers(spawn, numMidTiers, baseObject);

                //Spawn a roof
                this.SpawnTopTier(spawn, lastMidTier);
            }
        }

    }

}