1) Either a place where salt was made or refined (OED) or, more usually, a building where it was stored.
1503 To my wif a meise with one salt howse in Whitby, York
1528 M John Langton for a salthows wiche paid at a terme vs now iijs iiijd, Hull
1542 To John Maister ... my salte house with the newe tymbre that I bought for it, Whitby. Salthouse Lane in Hull takes its name from such a building: 1465 ‘the decay of a tenement called Salthous’
1465 the Comon seuer in Salthouslane by yere ijs.