Generating consistently-dimensioned polygons in mm units?

Your algorithm makes sense, but it seems that your problem is due to a rounding error when you divide by 2000 (divide by integer, which explains why a number smaller than two gives 0, and all distances are rounded to even values)

You should change the integer division with a float division

l = length / 2000

should be

l = length / 2000. # the . makes sure that you divide by a decimal value

or

l = float(length) / 2000

Note that this gives you the exact dimensions entered by the form, but you could decide to round the size of your parcels at one meter if you prefer:

l = float(length/1000) / 2

Note that you should also check the rounding at the start coordinates, but I do not know if this rounding is on purpose.

start_x = bbox.xMinimum() + float(distance_x) / 2

Thanks to @radouxju, here is the final code which also takes into account the horizontal and vertical distances being zero:

from PyQt4.QtCore import QVariant
from math import ceil

def generate_pv_panels(length, width, distance_x, distance_y):
    # Define layer properties
    layer = iface.activeLayer()
    crs = layer.crs()
    memory_lyr = QgsVectorLayer("Polygon?crs=epsg:" + unicode(crs.postgisSrid()) + "&index=yes", "PV panels for " + str(layer.name()), "memory")
    QgsMapLayerRegistry.instance().addMapLayer(memory_lyr)
    memory_lyr.startEditing()
    provider = memory_lyr.dataProvider()
    provider.addAttributes([QgsField("ID", QVariant.Int)])
    # Define variables
    fid = 0
    start_x = 0
    start_y = 0
    state_x = False
    state_y = False
    # Ensure polygons are not created 'within each other' if distance is zero;
    # Instead they will align on the bounding box
    if distance_x == 0:
        distance_x = (length / 1000)
        state_x = True
    if distance_y == 0:
        distance_y = (width / 1000)
        state_y = True
    fts = []
    for f in layer.getFeatures():
        fid += 1
        bbox = f.geometry().boundingBox()
        start_x = bbox.xMinimum() + float(distance_x / 2)
        start_y = bbox.yMinimum() + float(distance_y / 2)
        for row in range(0, int(ceil(bbox.height() / distance_y))):
            for column in range(0, int(ceil(bbox.width() / distance_x))):
                fet = QgsFeature()
                geom_type = pv_panel_size(length, width, start_x, start_y)
                if f.geometry().contains(geom_type):
                    fet.setGeometry(geom_type)
                    fet.setAttributes([fid])
                    fts.append(fet)
                if state_x == False:
                    start_x += distance_x + (length / 1000)
                else:
                    start_x += distance_x
            start_x = bbox.xMinimum() + float(distance_x / 2)
            if state_y == False:
                start_y += distance_y + (width / 1000)
            else:
                start_y += distance_y
    provider.addFeatures(fts)
    memory_lyr.updateFields()
    memory_lyr.commitChanges()

def pv_panel_size(length, width, x, y):
    # Length & width measured in mm; x & y measured in m
    l = float(length) / 2000
    w = float(width) / 2000
    return QgsGeometry.fromRect(QgsRectangle(x - l, y - w, x + l, y + w))

---

• Using generate_pv_panels(5500, 5000, 20, 1):

  

---

• Using generate_pv_panels(5500, 5000, 20, 0):