Mesh Networks and Ethernet
It can be difficult to provide adequate Wi-Fi coverage in larger homes. You can overcome this by using Ethernet, multiple access points, or Mesh Wi-Fi systems.
Generally speaking, if you have the option to connect something via Ethernet, whether it is a wireless access point, or an AppleTV, do it. It will provide a faster, more reliable connection and will reduce the amount of traffic on your Wi-Fi network. For new construction or remodeling, consider running Ethernet to all locations that you would place AV/gear (speakers, TVs, etc), wireless access points, security cameras, doorbells, thermostats, and smoke detectors. Note that even if you don't use Ethernet in some of those locations, you can still take advantage of the wires in an Ethernet cable to connect other low-voltage systems.
Multiple Access Points and Mesh:
You can install multiple access points to extend the range of a Wi-Fi network. This can be done a number of different ways, but you should be aware that they are not all created equal. Read Wirecutter Reviews for help in selecting a Wi-Fi system, and to better understand how Wi-Fi works.
Wi-Fi Extenders: The simplest method is to buy an additional Wireless Access Point (WAP) designed to extend your existing Wi-Fi network. This is the least efficient, because it is sharing the wireless radio bandwidth between the wireless clients (your phones, laptops and smart devices) and your main Wi-Fi router. You can also run into potential issues with roaming and mDNS communications, depending on the capabilities of your network devices and how they are configured. Finally, extended networks are managed separately from the main network -it's an add-on device- each device has to be configured and updated using different software and as separate units.
Mesh Wi-Fi: You can also extend your wireless Wi-Fi coverage by installing a Mesh Wi-Fi System, which consists of a main Wi-Fi Router Access Point ("base station") and one or more Wi-Fi "Satellite" Access Points. The wireless access points connect to each other, making a large network. The difference between a mesh system and an extended system is that all of the access points are designed to work together, which helps with things like roaming and mDNS. Since it is one system, you only have to configure and manage a single router. Mesh systems have better awareness of all of the network devices in your home, making their interaction more seamless. This is quickly becoming one of the most popular methods of expanding wireless coverage in the home because it is relatively simple to install and provides much better performance over using wireless extenders. Not all mesh systems are created equal. When looking at mesh systems, be sure to look for one that uses a dedicated radio for the backhaul (the link that connects each access point to another) connections. Some also provide an option to use Ethernet for the backhaul connections (see below).
Ethernet Backhaul: The third method to create a large Wi-Fi coverage area is to purchase multiple Wireless Access Points (usually designed to work together) and connect them via Ethernet cabling to a central network switch. This type of installation requires a wireless controller to make all of the access points work together (technically you can build a multiple WAP wireless network without a controller, but that is a more advanced network to manage, and you better know what you are doing!). This controller may be a separate piece of equipment, or it may be built into the main Wi-Fi router.
What I Use
I use the NetGear Orbi RBK50 Mesh Wi-Fi System. It has proven very reliable and offers good overall performance. As I stated earlier, it isn't at all uncommon for multiple video and audio streams to be going on at the same time in my home, and my HomeKit devices have always performed reliably and quickly. This doesn't mean that other Wi-Fi systems won't work, this is just what I use, and I have found it to be very reliable for home Wi-Fi in general and HomeKit devices specifically.