The landlord can not simply walk in (unless there is an emergency), but
he can enter on very short notice if he is showing the property to an insurance estimator, real estate appraiser, contractors or exterminators. Some municipalities also allow for routine inspections by fire marshals and building code enforcement officers.
You might also consider that you will be held responsible for any damage you cause to the house - water damage to the floor and mold behind the walls are extremely costly to repair.
If your operation is discovered, the house may be regarded as a "
stigmatized property", which could affect it's value and have the owner seeking a remedy from you in a civil court.
Consider all the ramifications and potential liability - then proceed with caution.