When quoting for jobs I offer either an all in quote for labor or a daily rate.  Longer, bigger jobs usually require a fixed price.  Clients will often want to provide materials, but I will orgainise this if necessary.  All quotes are VAT free (as I am not required to be VAT registered in France).  

In France, a quote is a contract to do the listed jobs for an agreed price.  Normally, the devis/quote should be signed by both parities.  Often (especially on larger jobs) things will be added on or a client may change his/her mind about what is to be done.  In this case, I re-write the quote adding on any extras in labor.  Overall, the French devis system is to avoid ambiguity or conflict – said work is done for said price.  Most quotes require around 30% up front, although larger jobs need a scheduled payment program.  

Certain jobs may need to be sub-contracted to different tradesmen (plumbers, electricians, diggers etc).  In this case, I always ask the clients accord or for their own traders where possible.

Working at someones house is a question of confidence (who is going to be on-site, when and with whom).  Money is also ex- changed, thus all parties should know exactly what is expected of them.  Overall, my aim is for a win-win situation, happy client, happy trader.