The most cost effective way to self build a garden office is to start with a good quality garden shed with double glazed doors and windows. It is imperative that the external cladding is tanalised  (pressure treated) as this will enhance the life expectancy, and you must have a vapour barrier to stop any external ingress of water, should the cladding get wet. You will need sufficient framing so that at least 50mm of insulation can be added to the walls, usually fitted within the cavity to ensure the internal dimensions are kept to the maximum.

The height of the walls will be determined by the shape of the roof, if you are keeping the building at 2.5m for permitted development purposes. An apex or pitched roof will need a minimum of 15 degree pitch if you are fitting felt tiles, cedar shingles, or lightweight slates. Standard mineral felt can accept a lesser pitch, on an apex roof, as can steel roof sheets.

Single slope (pent roof) buildings usually have metal roof sheets or EPDM roof covering, as the slope is between 1 and 5 degrees. 

On all roof versions, we would recommend an underlay felt as a base layer, as it allows the cold roof system to work correctly, by wicking away any moisture, whilst adding another protective outer skin.

Cables for the lighting can be 1.5mm twin & earth wires, and must be double insulated. The easiest solution is to install a wire from the fuse board to the switch, then from the switch the first light, and from the first light to the next light and so on. If you are having two circuits, you will need a second wire from the switch to the second string of lights. The end of the wiring finishes as the last light (no deed to go back to the switch). Simply position the cable to each light location and leave enough slack cable hanging down in a loop so that the luminaire can be fitted easily. LED panels or downlights are suitable for the lighting - however downlights usually need 60mm clearance above the back of the light for safety reasons.

Again - an electrician should do the final connections

Whist the DIY enthusiast usually likes to go it alone, there needs to be at least two people to carry and stall the basic body of the building. Once the walls are up and the roof boxes are on, the heavy lifting is all but over, and the craftsman can continue on his way to finish the project. Using the self build kit design means the majority of buildings can be watertight within 1 day, leaving the internal fitting out to be done over a couple of days. 

For self build purposes, polystyrene is acceptable (providing electric cables don't contact it)- this is known as Jablite. Alternatives such as Celotex, or Rockwool are also acceptable, however Rockwool is not as effective as the Celotex equivalent for insulation purposes, as it is not as densely packed together, however it is a cheaper option.

Insulation in the floor is a must, as it protects against the solid base passing cold through into the building from underneath. You will also need to leave an air gap between the roof boards and the insulation to prevent condensation, as the office will have a cold roof system.   

Once the insulation has been added to the floor, walls and ceiling, you can install the cables and other services within the cavity. Determine where the power feed (armoured cable) will be located, and decide where the sockets are to be installed. Starting from the fuse board, install 2.5mm double insulated (twin & earth) from the consumer unit to the first socket, then onto to the next socket, before finally finishing back at the fuse board to create a ring main. Each cable should be installed in safe zones, with enough slack at the sockets to cut the cables and fix to the faceplates. You will need an electrician to check the wiring and connect the faceplates. Safe zones run in vertical or horizontal directions from the sockets (not wavy or diagonal).

Once the services have been fitted, the internal lining boards can be added. 

The most cost effective solution is MDF, however vertical tongue and groove will offer a much neater finish, as the joins blend in and become part of the natural vertical lines. For those that accept the vertical joins, plasterboard is the cheapest, followed by MDF - both of which can also be fitted to the ceiling. 

Once the walls and ceiling are lined, cut out the holes for the back boxes so that the wiring can be finished. Then add the flooring (if required) before fitting the skirting, coving and any other corner or window trims. To finish, simply decorate, or plaster the inside and decorate.

For a more natural look, grooved lining boards are the most cost effective solution for lining the walls, along with plywood for the ceiling.

Buying a self build garden office kit may well be a cost effective solution to you, however to ensure that the building lasts, it needs a ground supported base. This can either be a concrete pad or paving slab base to the external size of the building. 75mm of hardcore and concrete is usually for buildings under 15 square metres - otherwise 150mm hardcore and 100mm concrete is preferred. Alternatively ground screws are an option, which are easier to install, but require larger floor timbers.