Best Smart Heating Control for a Granny Annexe: 5 Picks That Keep an Annex Warm Without Heating the Whole House

An annexe needs its own thermostat, not a share of the main house’s schedule. The five picks here all run a separate heating zone over Wi-Fi, so the annexe warms on its own timetable and the main house stays untouched. They suit three kinds of buyer: the family fitting out a self-contained annexe for a relative, the landlord who wants remote control of a rental unit, and the DIY installer who wants a quick swap with no rewiring drama. The overview below sorts them by price tier and best use, then each pick gets a full breakdown.

Smart Heating Controls for an Annexe Compared

Product	Key Spec	Best Use	Price Tier
Honeywell Home RTH9585WF1004	7-day programmable, color touchscreen	Full weekly scheduling with a big, readable display	Mid
Sensi Smart Thermostat ST55	DIY Wi-Fi, Energy Star certified	Budget-friendly basic zoning	Budget
ecobee Smart Thermostat Enhanced	Works with Siri, Alexa, Google Assistant	Voice-first control in a modern annexe	Upper mid
ecobee Smart Thermostat Essential	Programmable Wi-Fi, Energy Star certified	Value ecobee features for a simple install	Mid
Amazon Smart Thermostat	Works with Alexa and Ring	Alexa households wanting the lowest outlay	Budget

1. Honeywell Home RTH9585WF1004 Wi-Fi Smart Color Thermostat

Type: Wi-Fi smart thermostat
Programming: 7-day programmable
Display: Color touch screen
Voice control: Alexa ready
Certification: Energy Star
Finish: Gray

Honeywell Home RTH9585WF1004 Wi-Fi Smart Color Thermostat is the pick for anyone who wants to set a full week of annexe heating by hand and then leave it alone. The color touchscreen is the standout feature: large, bright and easy to read across a small room, which matters in an annexe where the thermostat often sits in the main living space. The 7-day programming means every day of the week gets its own schedule, useful when an annexe is occupied on some days and empty on others. Alexa readiness adds voice control for anyone who already runs an Echo in the annexe, and the Energy Star listing signals efficient scheduling behaviour. Build quality sits at the solid end of the mid-market, with a gray casing that blends into neutral walls.

This is the pick for an annexe with a fixed weekly routine — a relative who is home most mornings and out on certain afternoons. The big display also suits older users who find small screens fiddly. Where it falls short is app-first convenience: the touchscreen is the main event, so a buyer who wants to run everything from a phone will find the experience less polished than a voice-centric rival.

Trade-off: A screen-led interface means more time spent at the wall unit and less in the app, which will frustrate buyers who expect phone-first control.

Check the current price on Amazon

2. Sensi Smart Thermostat ST55

Type: Wi-Fi smart thermostat
Installation: DIY
Voice control: Alexa
Certification: Energy Star
Model: ST55

Sensi Smart Thermostat, Wi-Fi, DIY, Alexa, Energy Star Certified, ST55 is the budget entry and the one built around self-installation. The DIY label is the point: this is a thermostat designed to be swapped in by a competent householder rather than a contractor, which keeps the total cost of fitting out an annexe down. Wi-Fi control covers the essentials — remote adjustment, scheduling and a phone app — without paying for extra sensors or a premium screen. Alexa support gives voice control for those already in that ecosystem, and the Energy Star certification keeps running costs in check. The casing is plain and functional, the sort of unit that disappears on a hallway wall.

This pick suits the annexe that needs reliable, no-frills smart heating on a tight budget — a conversion where the heating system is simple and the user just wants the annexe warm when occupied. It is not for the buyer who wants a large display or a touchscreen interface, and the feature set is deliberately basic.

Trade-off: The low price buys a plain interface and a lean feature set, so buyers wanting room sensors or a premium display will need to spend more.

Check the current price on Amazon

3. ecobee Smart Thermostat Enhanced

Type: Programmable Wi-Fi smart thermostat
Voice control: Siri, Alexa, Google Assistant
Certification: Energy Star
Series: Enhanced
Category: Smart home

ecobee Smart Thermostat Enhanced, Programmable Wifi Thermostat is the voice-control champion of the group, and that is exactly what makes it a strong fit for a granny annexe. All three major assistants are supported — Siri, Alexa and Google Assistant — so voice commands work whichever speaker the annexe already has. That matters more in an annexe than a main house: a relative with limited mobility can adjust the heating without walking to the wall unit. The Enhanced labelling points to a step up in hardware and sensor capability over the base model, and the Energy Star certification keeps efficiency claims grounded. Programming is built in for a weekly schedule, with app control as the primary route.

This is the pick for a modern, voice-enabled annexe and for anyone who values hands-free adjustment. It is not the budget choice, and buyers who never use a voice assistant will be paying for capability they won’t touch.

Trade-off: The premium voice and sensor package pushes the price up, so a voice-free household pays for features it will never use.

Check the current price on Amazon

4. ecobee Smart Thermostat Essential

Type: Programmable Wi-Fi smart thermostat
Voice control: Siri, Alexa, Google Assistant
Certification: Energy Star
Series: Essential
Category: Energy saving thermostat

ecobee Smart Thermostat Essential – WiFi Thermostat takes the ecobee formula and strips it back to the essentials, which is often the right answer for an annexe. The same three-assistant support is here — Siri, Alexa and Google Assistant — so voice control is not sacrificed, and Energy Star certification is retained. What the Essential badge signals is a leaner package than the Enhanced: fewer extras, lower cost, the core smart-thermostat job done properly. Wi-Fi programming and app control cover the scheduling needs of a single-zone annexe without overkill. For a buyer who wants ecobee reliability but does not need the top-tier hardware, this is the sensible middle path.

It suits the practical annexe owner who wants voice control and efficiency without the flagship price. It is not for the buyer chasing the most advanced sensor features — that is what the Enhanced is for.

Trade-off: Stripping back to essentials means fewer of the advanced sensors and extras that the Enhanced model carries, so the budget saving is real but not free.

Check the current price on Amazon

5. Amazon Smart Thermostat

Type: Smart thermostat
Voice control: Alexa
Integration: Ring
Category: Energy saving

Amazon Smart Thermostat, Save money and energy, Works with Alexa and Ring is the lowest-cost route into smart heating and the natural choice for a household already living inside the Amazon ecosystem. Alexa control is native rather than bolted on, and Ring integration means the annexe thermostat can sit alongside doorbells and cameras in one app. The "save money and energy" positioning points at scheduling and away-mode behaviour that trims running costs in a space that is empty for long stretches. The hardware is deliberately simple — no color screen, no premium finish — which keeps both the price and the failure points down.

This pick suits the annexe attached to a tech-savvy main house where Alexa and Ring already run the show, and where the user wants the cheapest credible smart thermostat. It is not for Siri or Google households, and it is not for anyone who wants a display-led interface.

Trade-off: The budget price and Alexa-only voice support lock the buyer into Amazon’s ecosystem, so Apple and Google homes should look elsewhere.

Check the current price on Amazon

Before You Buy

• Compatibility: Check the annexe’s heating system type — the thermostat must match the wiring and fuel type already installed before any purchase.
• Zoning: Confirm the annexe runs as its own heating zone; a thermostat cannot separate it from the main house if the pipework is shared.
• Voice ecosystem: Match the thermostat to the assistants already in use — Alexa for the Amazon pick, all three for the ecobee models, Alexa for Honeywell and Sensi.
• Installation: The Sensi is explicitly DIY; the others may need a competent installer if the existing wiring is unfamiliar.
• Connectivity: The annexe needs a Wi-Fi signal strong enough at the thermostat’s wall position, which can be a weak spot in converted outbuildings.
• Scheduling needs: A 7-day model like the Honeywell suits irregular occupancy; simpler models suit a set weekly routine.
• Budget: Set the budget around the full cost of thermostat plus any installation, not the unit price alone.

Frequently Asked Questions

1. Can a smart thermostat heat an annexe separately from the main house?
   Yes, provided the annexe is on its own heating zone with its own valves or circuit. The thermostat then controls that zone alone.

2. Which pick is best for an older relative with limited mobility?
   The ecobee Smart Thermostat Enhanced, because Siri, Alexa and Google Assistant support allows hands-free adjustment from a chair or bed.

3. Do these thermostats work without a subscription?
   Core scheduling, app control and voice commands work without a subscription on all five; premium analytics services, where offered, are optional extras.

4. What if the annexe has weak Wi-Fi?
   A mesh node or extender near the annexe is usually the fix; without a stable signal, app and voice control will be unreliable regardless of the thermostat chosen.

5. Is the cheapest option good enough for an annexe?
   The Amazon Smart Thermostat covers scheduling and Alexa control at the lowest outlay, which is enough for a simple single-zone annexe in an Amazon household.

6. Do I need a professional to install one?
   Not necessarily — the Sensi is designed for DIY — but unfamiliar wiring or a combi boiler setup is best left to a qualified heating engineer.

Bottom Line

For most granny annexes the ecobee Smart Thermostat Enhanced is the strongest all-round choice: it works with all three voice assistants, carries Energy Star certification and gives a relative hands-free control without leaving the chair. Buyers on a tight budget in an Alexa household should take the Amazon Smart Thermostat instead, and anyone who prefers a big, readable wall screen will be happiest with the Honeywell Home RTH9585WF1004. Whichever fits, check the current price before ordering.