No Children, No Pets (Knopf, 1956), by Marion Holland

Written and illustrated by Marion Holland, 'No Children, No Pets' was a Weekly Reader Children's Book Club selection in February 1957.
From the inside front cover of the Alfred A. Knopf edition:
"It was a very hot August day when the registered air mail letter arrived--the letter that sent the entire Sanders family (Mother, Don and Jane, four-year-old Betsy, and Victoria the cat) off to Palm Glade, Florida, where they had just inherited an apartment house!
"Everybody looked forward to a good rest, but they reckoned without the strange tenants at 303 Oleander Drive. Little did Don or Jane dream (or Mother either, for that matter) that they would soon become involved not only with a hurricane but also with Miss Giddings's carefully guarded secret, Mrs. Pennypacker's ruby clip, a missing superintendent, and a strange and homeless boy.
"The author's own illustrations add just the right humorous touch to a story that will delight and intrigue both boys and girls."
From the inside front cover of the Alfred A. Knopf edition:
"It was a very hot August day when the registered air mail letter arrived--the letter that sent the entire Sanders family (Mother, Don and Jane, four-year-old Betsy, and Victoria the cat) off to Palm Glade, Florida, where they had just inherited an apartment house!
"Everybody looked forward to a good rest, but they reckoned without the strange tenants at 303 Oleander Drive. Little did Don or Jane dream (or Mother either, for that matter) that they would soon become involved not only with a hurricane but also with Miss Giddings's carefully guarded secret, Mrs. Pennypacker's ruby clip, a missing superintendent, and a strange and homeless boy.
"The author's own illustrations add just the right humorous touch to a story that will delight and intrigue both boys and girls."