To use a house / flat officially as an office you would need to apply for planning permission for change of use. There may also be restricted covenants in the development which would prevent it.
Comparing office prices is quite difficult – especially if you are looking at short term serviced office arrangements:
– rates
– electricity (and gas)
– water (sewage and “roof” drainage are extras)
– phone lines
– broadband
– reception facilities
– desks / furniture
– notice periods
– cleaning
– service charges for communal spaces
– liability for repairs
– meeting room charges
– post charges
– security passes / access cards
– tea/coffee facilities
…etc.
are all highly variable from place to place, and what is / isn’t included in the price. It may be you aren’t comparing like with like.