Games like Undiscovered House
Games like Undiscovered House include Poppy Playtime, Outlast, Phasmophobia, Garten of Banban, and MiSide. We matched them by the tags players give Undiscovered House on Steam (Horror, Indie, First-Person, Dark, and 3D): closest matches first, and among equally close ones the better-known games.

Poppy Playtime
Shared: Horror · First-Person · Dark

Outlast
Shared: Horror · First-Person · Atmospheric

Phasmophobia
Shared: Horror · First-Person · Dark

Garten of Banban
Shared: Horror · First-Person · 3D

MiSide
Shared: Horror · First-Person · Psychological Horror

No, I'm not a Human
Shared: Horror · Indie · Atmospheric

Buckshot Roulette
Shared: Horror · Dark · First-Person

Firewatch
Shared: Atmospheric · First-Person · Mystery

SOMA
Shared: Horror · Atmospheric · Dark

Mouthwashing
Shared: Horror · Indie · Atmospheric

Layers of Fear (2016)
Shared: Horror · First-Person · Atmospheric

What Remains of Edith Finch
Shared: First-Person · Atmospheric · Mystery

Outlast 2
Shared: Horror · First-Person · Dark

The Room Two
Shared: Horror · Mystery · Atmospheric

Bendy and the Ink Machine
Shared: Horror · First-Person · Indie

Lethal Company
Shared: Horror · First-Person · Indie

From The Darkness
Shared: Horror · Indie · Dark

Escape the Backrooms
Shared: Horror · First-Person · Psychological Horror

Garten of Banban 2
Shared: Horror · First-Person · Dark

Psalm 5:9-13
Shared: Horror · Indie · Mystery

Little Nightmares II
Shared: Horror · Dark · Atmospheric

Slender: The Arrival
Shared: Horror · First-Person · Indie

Sally Face - Episode One
Shared: Indie · Horror · Dark

Five Nights at Freddy's: Secret of the Mimic
Shared: Horror · First-Person · 3D