(第)一层 means first floor here

you can see the pictures to understand 层 http://image.baidu.com/search/index?tn=baiduimage&ipn=r&ct=201326592&cl=2&lm=-1&st=-1&fm=result&fr=&sf=1&fmq=1480775086933_R&pv=&ic=0&nc=1&z=&se=1&showtab=0&fb=0&width=&height=&face=0&istype=2&ie=utf-8&word=三层 http://image.baidu.com/search/detail?ct=503316480&z=&tn=baiduimagedetail&ipn=d&word=千层蛋糕&step_word=&ie=utf-8&in=&cl=2&lm=-1&st=-1&cs=3083553270,1922073177&os=2188278986,4129026289&simid=4101175270,528792869&pn=14&rn=1&di=13438289040&ln=1981&fr=&fmq=1480775108907_R&fm=result&ic=0&s=undefined&se=&sme=&tab=0&width=&height=&face=undefined&is=0,0&istype=2&ist=&jit=&bdtype=0&spn=0&adpicid=0&pi=0&gsm=0&objurl=http%3A%2F%2Fimg3.cache.netease.com%2Fhebei%2F2015%2F5%2F17%2F201505171712173467e_550.png&rpstart=0&rpnum=0&adpicid=0&ctd=1480775121369^3_1347X637%1

It means the first floor. AKA, 一楼。

一层 means floor I live on the second floor 我住在第二层楼

the receptionist is meeting with a client at the office building in the level one or ground floor