Around a dozen tenancies - beyond that a column of buttons is hard to scan and hard to maintain.

With a small number of dwellings, a button per dwelling with the name beside it is the clearest possible interface. A visitor sees the name and presses the adjacent button, with no instructions needed.

As the count rises, the panel grows into multiple columns, the labels shrink, and finding a name takes time - at night, in the rain, often by someone unfamiliar with the building.

Relabelling becomes a problem too. Changing one name in a printed insert strip means reprinting the strip.

A digital directory shows a scrollable list with a search, and the visitor selects or dials a code. Updates are made in software, so a change of occupant takes seconds.

The crossover point depends on turnover as much as size: a building with frequent tenant changes benefits from a digital directory at a lower count than one with long-term residents.

Audio or video units, button columns, digital directories, keypads, readers, information displays and blank filler modules - assembled into a frame.

The audio or video module is the core, containing the microphone, speaker and camera.

Button modules come in fixed counts and are stacked to reach the required number, with labelling built in.

A directory module provides the scrollable list and its selection controls.

Keypad modules allow entry by code, which serves residents and also lets visitors dial a dwelling number directly.

Reader modules integrate access control credentials into the panel, so residents present a card or fob rather than calling.

Display modules show instructions, building information or call status.

Blank modules fill unused frame positions and are what allow future expansion - specifying a frame with spare positions and blanks is much cheaper than replacing the frame later, and should be standard practice.

More than expected in every dimension - and depth is the constraint that causes problems on site.

The frame's height grows with the number of modules, and a panel for a large block can be substantial. That has to fit the available wall, within the reach range accessibility requires, which is a real constraint: the modules people need to operate must sit within the reachable band while the frame as a whole may extend beyond it.

Depth is where installations fail. Flush back boxes for modular panels are deep, and a stud partition, a cavity wall with insulation, or a thin masonry return may not accommodate one. Establish the wall construction and available depth before specifying flush mounting.

Weight matters on lightweight partitions and needs proper support.

And allow working space behind for the cabling and terminations - a panel packed so tightly that modules cannot be removed for service is a maintenance problem for the life of the building.

Yes if the frame has spare positions - which is why specifying spare capacity at installation is worth doing.

Adding a module to a frame with a blank position is straightforward: remove the blank, fit the module, configure it.

Adding beyond the frame's capacity means a new frame and a new back box, which in a flush installation means building work at the entrance of an occupied building - disruptive and expensive out of proportion to the module itself.

So specify a frame with room for growth. Buildings are subdivided, commercial units are split, and a panel sized exactly to today's occupancy will need extending.

Check the system's addressing capacity too. The controller and the wiring have limits on the number of dwellings supported, and physical space on the panel is only useful if the system behind it can address the additional units.

Record the frame model and available positions in the building's records, so a future change is a known quantity.

Stainless steel and anodised aluminium predominantly, with the choice driven by environment and by appearance.

Stainless is the standard for exposed and public positions: durable, resistant to impact, and available in grades suited to coastal environments where a lower grade would stain.

Anodised aluminium is lighter and available in a range of finishes, suiting sheltered positions and buildings where appearance is a priority.

Brass and bronze appear on heritage and high-specification buildings.

Engraving is the usual method of permanent labelling and is far more durable than printed inserts, at the cost of being fixed - which is fine for building names and instructions and wrong for occupant names.

In coastal and industrial atmospheres, specify the grade rather than the material alone, and use compatible fixings. A stainless panel with plated steel screws corrodes at the screws, which is both unsightly and eventually a removal problem.

Through a digital directory where possible, and through easily replaceable inserts where not - because names change and a panel that makes changing them difficult ends up with handwritten labels.

A digital directory is updated in software. A change of occupant is a database edit, and the panel shows the new name immediately. For a building with turnover, this is the single strongest argument for a directory.

Where printed inserts are used, they should be replaceable from the front or with minimal dismantling, weatherproof, and illuminated. Inserts that require the panel to be removed from the wall will not be changed.

Legibility deserves attention: adequate text size, good contrast, and illumination that works. Faded inserts behind scratched covers are the normal state of a neglected panel.

Whoever manages the building needs the means and the instructions to make changes. A system where only the installer can update names generates repeat call-outs and, eventually, tape.

Yes, through reader modules - and integrating them with the building's access control is usually better than running two separate credential systems.

A reader module in the panel lets residents and staff present a card, fob or phone credential and enter without calling anyone. That is how most people use the entrance most of the time.

The integration question is where credentials are managed. A reader that is part of the door entry system has its own database; one integrated with the access control system shares the building's credentials, permissions and audit trail - so a lost card is revoked once and access to the entrance and to internal doors is managed together.

The latter is preferable wherever access control exists.

Check credential technology compatibility: older proximity technologies are widely deployed but weak, and newer encrypted credentials are the current standard. A new panel is a good opportunity to move, but only if the whole estate moves with it.