replacing cistern valve without turning the water off?

As wirecutterman says for mains fed system.
But am a bit concerned at your meaning of 'New House' new build or just moved to another,but old house?

Check the attic for a large 50gal storage cold water tank you may find the bathroom cold feeds fed from this tank. If so some have stop valves fitted in the out flow pipe if the installation plumbers brain was working not like the one who fitted ours.
If no tank then follow wirecutterman.
But a warning about access to street stop valve you must contact the water authority to see what the position is for you to access the street stop valve and you will need a special stand pipe key in most I've been in to [of course meter fed may be different and easy to turn off]. You may also have the liablity to pay for repairs/replacement IF you break the stopvalve shaft if it is a very old one.

Bob
April 2010

somewhere just in side your property you will have a incoming (mains cold water) supply, turn off the stop cock on this, if this tap fails, need to find then turn off the one in the street/cul desac
may first need to talk to your neighbours, as you may share the stop tap and not know it, funny things happen when houses are built

wirecutterman
April 2010