Green’s Dictionary of Slang

1. the house where one’s mistress was kept, or which either sex kept for secret assignations.

1607		Dekker & Webster Northward Hoe I ii: Doe you not think this minx is some naughty packe whome my husband hath fallen in loue with, and meanes to keep [...] at his garden house.
1616–20		Middleton Mayor of Quinborough (1661) III i: She’s entic’d forth by her own Sex To be betray’d to man, who in some Garden-house, [...] taking his lustful time, Binds darkness on her eye-lids, surprizes her.
1626		H. Parrot Cures for the Itch F8: [Her husband] denyes accesse vnto her Garden-house.
1640		H. Mill Nights Search I 179: He at her garden house do’s find her out.
1674		Head Canting Academy (2nd edn) 50: Having occasion to be over to the Bank-side, in a Garden house.

2. a brothel.

1984		Partridge DSUE (8th edn) 446/2: C.17 [...] C.18–early 19.
2010		(con. 17C) W. Ings ‘Trolling the Beat to Working the Soob’ in Int’l Jrnl Lexicog. 23:1 62: Covent Garden was an area of London frequented by whores and the location gave rise to terms like garden house (brothel).